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A POPULAR HISTORY OF ASTRONOMY DURING THE NINETEENTH CENTURY
* * * * *
BY THE SAME AUTHOR
PROBLEMS IN ASTROPHYSICS.
Demy 8vo., cloth. Containing over 100
Illustrations. Price 20s. net.
THE SYSTEM OF THE STARS.
Second Edition. Thoroughly revised and
largely rewritten. Containing numerous
and new Illustrations. Demy 8vo., cloth.
Price 20s. net.
MODERN COSMOGONIES. Crown
8vo., cloth. Price 3s. 6d. net.
A. AND C. BLACK, SOHO SQUARE, LONDON, W.
* * * * *
[Illustration: THE GREAT NEBULA IN ORION, 1883
_See p. 408_]
A POPULAR HISTORY OF ASTRONOMY DURING THE NINETEENTH CENTURY
by
AGNES M. CLERKE
[Illustration: JUPITER 1879
SATURN 1885]
London
Adam and Charles Black
1908
First Edition, Post 8vo., published 1885
Second Edition, Post 8vo., published 1887
Third Edition, Demy 8vo., published 1893
Fourth Edition, Demy 8vo., published 1902
Fourth Edition, Post 8vo., reprinted February, 1908
PREFACE TO THE FOURTH EDITION
Since the third edition of the present work issued from the press, the
nineteenth century has run its course and finished its record. A new era
has dawned, not by chronological prescription alone, but to the vital
sense of humanity. Novel thoughts are rife; fresh impulses stir the
nations; the soughing of the wind of progress strikes every ear. "The
old order changeth" more and more swiftly as mental activity becomes
intensified. Already many of the scientific doctrines implicitly
accepted fifteen years ago begin to wear a superannuated aspect.
Dalton's atoms are in process of disintegration; Kirchhoff's theorem
visibly needs to be modified; Clerk Maxwell's medium no longer figures
as an indispensable factotum; "absolute zero" is known to be situated on
an asymptote to the curve of cold. Ideas, in short, have all at once
become plastic, and none more completely so than those relating to
astronomy. The physics of the heavenly bodies, indeed, finds its best
opportunities in unlooked-for disclosures; for it deals with
transcendental conditions, and what is strange to terrestrial experience
may serve admirably to expound what is normal in the skies. In celestial
science especially, facts that appear subversive are often the most
illuminative, and the prospect of its advance widens and brightens with
each divagation enforced or permitted from the strait paths of rigid
theory.
This readiness for innovation has undoubtedly its dangers and drawbacks.
To the historian, above all, it presents frequent occasions of
embarrassment. The writing of history is a strongly selective operation,
the outcome being valuable just in so far as the choice what to reject
and what to include has been judicious; and the task is no light one of
discriminating between barren speculations and ideas pregnant with
coming truth. To the possession of such prescience of the future as
would be needed to do this effectually I can lay no claim; but diligence
and sobriety of thought are ordinarily within reach, and these I shall
have exercised to good purpose if I have succeeded in rendering the
fourth edition of _A Popular History of Astronomy during the Nineteenth
Century_ not wholly unworthy of a place in the scientific literature of
the twentieth century.
My thanks are due to Sir David Gill for the use of his photograph of the
great comet of 1901, which I have added to my list of illustrations, and
to the Council of the Royal Astronomical Society for the loan of glass
positives needed for the reproduction of those included in the third
edition.
London, _July_, 1902.
PREFACE TO THE FIRST EDITION
The progress of astronomy during the last hundred years has been rapid
and extraordinary. In its distinctive features, moreover, the nature of
that progress has been such as to lend itself with facility to
untechnical treatment. To this circumstance the present volume owes its
origin. It embodies an attempt to enable the ordinary reader to follow,
with intelligent interest, the course of modern astronomical inquiries,
and to realize (so far as it can at present be realized) the full effect
of the comprehensive change in the whole aspect, purposes, and methods
of celestial science introduced by the momentous discovery of spectrum
analysis.
Since Professor Grant's invaluable work on the _History of Physical
Astronomy_ was published, a third of a century has elapsed. During the
interval a so-called "new astronomy" has grown up by the side of the
old. One effect of its advent has been to render the science of the
heavenly bodies more popular, both in its needs and in its nature, than
formerly. More popular in its needs, since its progress now primarily
depends upon the interest in, and consequent efforts towards its
advancement of the general public; more popular in its nature, because
the kind of knowledge it now chiefly tends to accumulate is more easily
intelligible--less remote from ordinary experience--than that evolved by
the aid of the calculus from materials collected by the use of the
transit-instrument and chronograph.
It has thus become practicable to describe in simple language the most
essential parts of recent astronomical discoveries, and, being
practicable, it could not be otherwise than desirable to do so. The
service to astronomy itself would be not inconsiderable of enlisting
wider sympathies on its behalf, while to help one single mind towards a
fuller understanding of the manifold works which have in all ages
irresistibly spoken to man of the glory of God might well be an object
of no ignoble ambition.
The present volume does not profess to be a complete or exhaustive
history of astronomy during the period covered by it. Its design is to
present a view of the progress of celestial science, on its most
characteristic side, since the time of Herschel. Abstruse mathematical
theories, unless in some of their more striking results, are excluded
from consideration. These, during the eighteenth century, constituted
the sum and substance of astronomy, and their fundamental importance can
never be diminished, and should never be ignored. But as the outcome of
the enormous development given to the powers of the telescope in recent
times, together with the swift advance of physical science, and the
inclusion, by means of the spectroscope, of the heavenly bodies within
the domain of its inquiries, much knowledge has been acquired regarding
the nature and condition of those bodies, forming, it might be said, a
science apart, and disembarrassed from immediate dependence upon
intricate, and, except to the initiated, unintelligible formulæ. This
kind of knowledge forms the main subject of the book now offered to the
public.
There are many reasons for preferring a history to a formal treatise on
astronomy. In a treatise, _what_ we know is set forth. A history tells
us, in addition, _how_ we came to know it. It thus places facts before
us in the natural order of their ascertainment, and narrates instead of
enumerating. The story to be told leaves the marvels of imagination far
behind, and requires no embellishment from literary art or high-flown
phrases. Its best ornament is unvarnished truthfulness, and this, at
least, may confidently be claimed to be bestowed upon it in the ensuing
pages.
In them unity of treatment is sought to be combined with a due regard to
chronological sequence by grouping in separate chapters the various
events relating to the several departments of descriptive astronomy. The
whole is divided into two parts, the line between which is roughly drawn
at the middle of the present century. Herschel's inquiries into the
construction of the heavens strike the keynote of the first part; the
discoveries of sun-spot and magnetic periodicity and of spectrum
analysis determine the character of the second. Where the nature of the
subject required it, however, this arrangement has been disregarded.
Clearness and consistency should obviously take precedence of method.
Thus, in treating of the telescopic scrutiny of the various planets, the
whole of the related facts have been collected into an uninterrupted
narrative. A division elsewhere natural and helpful would here have been
purely artificial, and therefore confusing.
The interests of students have been consulted by a full and authentic
system of references to the sources of information relied upon.
Materials have been derived, as a rule with very few exceptions, from
the original authorities. The system adopted has been to take as little
as possible at second-hand. Much pains have been taken to trace the
origin of ideas, often obscurely enunciated long before they came to
resound through the scientific world, and to give to each individual
discoverer, strictly and impartially, his due. Prominence has also been
assigned to the biographical element, as underlying and determining the
whole course of human endeavour. The advance of knowledge may be called
a vital process. The lives of men are absorbed into and assimilated by
it. Inquiries into the kind and mode of the surrender in each separate
case must always possess a strong interest, whether for study or for
example.
The acknowledgments of the writer are due to Professor Edward S. Holden,
director of the Washburn Observatory, Wisconsin, and to Dr. Copeland,
chief astronomer of Lord Crawford's Observatory at Dunecht, for many
valuable communications.
London, _September_, 1885.
CONTENTS
_INTRODUCTION_
page
Three Kinds of Astronomy--Progress of the Science during the
Eighteenth Century--Popularity and Rapid Advance during the Nineteenth
Century 1
PART I
_PROGRESS OF ASTRONOMY DURING THE FIRST HALF OF THE NINETEENTH CENTURY_
CHAPTER I
FOUNDATION OF SIDEREAL ASTRONOMY
State of Knowledge regarding the Stars in the Eighteenth Century--
Career of Sir William Herschel--Constitution of the Stellar System--
Double Stars--Herschel's Discovery of their Revolutions--
His Method of Star-gauging--Discoveries of Nebulæ--Theory of their
Condensation into Stars--Summary of Results 9
CHAPTER II
PROGRESS OF SIDEREAL ASTRONOMY
Exact Astronomy in Germany--Career of Bessel--His _Fundamenta
Astronomiæ_--Career of Fraunhofer--Parallaxes of Fixed
Stars--Translation of the Solar System--Astronomy of the
Invisible--Struve's Researches in Double Stars--Sir John Herschel's
Exploration of the Heavens--Fifty Years' Progress 27
CHAPTER III
PROGRESS OF KNOWLEDGE REGARDING THE SUN
Early Views as to the Nature of Sun-spots--Wilson's Observations and
Reasonings--Sir William Herschel's Theory of the Solar
Constitution--Sir John Herschel's Trade-Wind Hypothesis--Baily's
Beads--Total Solar Eclipse of 1842--Corona and Prominences--Eclipse of
1851 52
CHAPTER IV
PLANETARY DISCOVERIES
Bode's Law--Search for a Missing Planet--Its Discovery by Piazzi--
Further Discoveries of Minor Planets--Unexplained Disturbance of
Uranus--Discovery of Neptune--Its Satellite--An Eighth Saturnian
Moon--Saturn's Dusky Ring--The Uranian System 71
CHAPTER V
COMETS
Predicted Return of Halley's Comet--Career of Olbers--Acceleration of
Encke's Comet--Biela's Comet--Its Duplication--Faye's Comet--Comet of
1811--Electrical Theory of Cometary Emanations--The Earth in a Comet's
Tail--Second Return of Halley's Comet--Great Comet of 1843--Results to
Knowledge 88
CHAPTER VI
INSTRUMENTAL ADVANCES
Two Principles of Telescopic Construction--Early Reflectors--Three
Varieties--Herschel's Specula--High Magnifying Powers--Invention of
the Achromatic Lens--Guinand's Optical Glass--The Great Rosse
Reflector--Its Disclosures--Mounting of Telescopes--Astronomical
Circles--Personal Equation 108
PART II
_RECENT PROGRESS OF ASTRONOMY_
CHAPTER I
FOUNDATION OF ASTRONOMICAL PHYSICS
Schwabe's Discovery of a Decennial Sun-spot Period--Coincidence with
Period of Magnetic Disturbance--Sun-spots and Weather--Spectrum
Analysis--Preliminary Inquiries--Fraunhofer Lines--Kirchhoff's
Principle--Anticipations--Elementary Principles of Spectrum
Analysis--Unity of Nature 125
CHAPTER II
SOLAR OBSERVATIONS AND THEORIES
Black Openings in Spots--Carrington's Observations--Rotation of the
Sun--Kirchhoff's Theory of the Solar Constitution--Faye's Views--Solar
Photography--Kew Observations--Spectroscopic Method--Cyclonic Theory
of Sun-spots--Volcanic Hypothesis--A Solar Outburst--Sun-spot
Periodicity--Planetary Influence--Structure of the Photosphere 143
CHAPTER III
RECENT SOLAR ECLIPSES
Expeditions to Spain--Great Indian Eclipse--New Method of Viewing
Prominences--Total Eclipse Visible in North America--Spectrum of the
Corona--Eclipse of 1870--Young's Reversing Layer--Eclipse of
1871--Corona of 1878--Varying Coronal Types--Egyptian
Eclipse--Daylight Coronal Photography--Observations at Caroline
Island--Photographs of Corona in 1886 and 1889--Eclipses of 1896,
1898, 1900, and 1901--Mechanical Theory of Corona--Electro-Magnetic
Theories--Nature of Corona 166
CHAPTER IV
SOLAR SPECTROSCOPY
Chemistry of Prominences--Study of their Forms--Two
Classes--Photographs and Spectrographs of Prominences--Their
Distribution--Structure of the Chromosphere--Spectroscopic Measurement
of Radial Movements--Spectroscopic Determination of Solar
Rotation--Velocities of Transport in the Sun--Lockyer's Theory of
Dissociation--Solar Constituents--Oxygen Absorption in Solar
Spectrum 194
CHAPTER V
TEMPERATURE OF THE SUN
Thermal Power of the Sun--Radiation and Temperature--Estimates of
Solar Temperature--Rosetti's and Wilson's Results--Zöllner's Method
--Langley's Experiment at Pittsburg--The Sun's Atmosphere--Langley's
Bolometric Researches--Selective Absorption by our Air--The Solar
Constant 216
CHAPTER VI
THE SUN'S DISTANCE
Difficulty of the Problem--Oppositions of Mars--Transits of
Venus--Lunar Disturbance--Velocity of Light--Transit of
1874--Inconclusive Result--Opposition of Mars in 1877--Measurements of
Minor Planets--Transit of 1882--Newcomb's Determination of the
Velocity of Light--Combined Result 227
CHAPTER VII
PLANETS AND SATELLITES
Schröter's Life and Work--Luminous Appearances during Transits of
Mercury--Mountains of Mercury--Intra-Mercurian Planets--Schiaparelli's
Results for the Rotation of Mercury and Venus--Illusory
Satellite--Mountains and Atmosphere of Venus--Ashen Light--Solidity of
the Earth--Variation of Latitude--Secular Changes of Climate--Figure
of the Globe--Study of the Moon's Surface--Lunar Atmosphere--New
Craters--Thermal Energy of Moonlight--Tidal Friction 243
CHAPTER VIII
PLANETS AND SATELLITES--(_continued_)
Analogy between Mars and the Earth--Martian Snowcaps, Seas, and
Continents--Climate and Atmosphere--Schiaparelli's Canals--Discovery
of Two Martian Satellites--Photographic Detection of Minor
Planets--Orbit of Eros--Distribution of the Minor Planets--Their
Collective Mass and Estimated Diameters--Condition of Jupiter--His
Spectrum--Transits of his Satellites--Discovery of a Fifth
Satellite--The Great Red Spot--Constitution of Saturn's Rings--Period
of Rotation of the Planet--Variability of Japetus--Equatorial Markings
on Uranus--His Spectrum--Rotation of Neptune--Trans-Neptunian
Planets 274
CHAPTER IX
THEORIES OF PLANETARY EVOLUTION
Origin of the World according to Kant--Laplace's Nebular
Hypothesis--Maintenance of the Sun's Heat--Meteoric
Hypothesis--Radiation as an Effect of Contraction--Regenerative
Theory--Faye's Scheme of Planetary Development--Origin of the
Moon--Effects of Tidal Friction 308
CHAPTER X
RECENT COMETS
Donati's Comet--The Earth again Involved in a Comet's Tail--Comets of
the August and November Meteors--Star Showers--Comets and
Meteors--Biela's Comet and the Andromedes--Holmes's Comet--Deflection
of the Leonids--Orbits of Meteorites--Meteors with Stationary
Radiants--Spectroscopic Analysis of Cometary Light--Comet of
1901--Coggia's Comet 323
CHAPTER XI
RECENT COMETS--(_continued_)
Forms of Comets' Tails--Electrical Repulsion--Brédikhine's Three
Types--Great Southern Comet--Supposed Previous Appearances--Tebbutt's
Comet and the Comet of 1807--Successful Photographs--Schaeberle's
Comet--Comet Wells--Sodium Blaze in Spectrum--Great Comet of
1882--Transit across the Sun--Relation to Comets of 1843 and
1880--Cometary Systems--Spectral Changes in Comet of 1882--Brooks's
Comet of 1889--Swift's Comet of 1892--Origin of Comets 345
CHAPTER XII
STARS AND NEBULÆ
Stellar Chemistry--Four Orders of Stars--Their Relative Ages--Gaseous
Stars--Spectroscopic Star-Catalogues--Stellar Chemistry--Hydrogen
Spectrum in Stars--The Draper Catalogue--Velocities of Stars in Line
of Sight--Spectroscopic Binaries--Eclipses of Algol--Catalogues of
Variables--New Stars--Outbursts in Nebulæ--Nova Aurigæ--Nova
Persei--Gaseous Nebulæ--Variable Nebulæ--Movements of Nebulæ--Stellar
and Nebular Photography--Nebulæ in the Pleiades--Photographic
Star-charting--Stellar Parallax--Double Stars--Stellar
Photometry--Status of Nebulæ--Photographs and Drawings of the Milky
Way--Star Drift 372
CHAPTER XIII
METHODS OF RESEARCH
Development of Telescopic Power--Silvered Glass Reflectors--Giant
Refractors--Comparison with Reflectors--The Yerkes
Telescope--Atmospheric Disturbance--The Lick Observatory--Mechanical
Difficulties--The Equatoreal _Coudé_--The Photographic
Camera--Retrospect and Conclusion 428
APPENDIX
Chronology, 1774-1893--Chemical Elements in the Sun (Rowland,
1891)--Epochs of Sun-spot Maximum and Minimum from 1610 to
1901--Movements of Sun and Stars--List of Great Telescopes--List of
Observatories employed in the Construction of the Photographic Chart
and Catalogue of the Heavens 443
INDEX 471
LIST OF ILLUSTRATIONS
Photograph of the Great Nebula in Orion, 1883 _Frontispiece_
Photographs of Jupiter, 1879, and of Saturn, 1885 _Vignette_
Plate I. Photographs of the Solar Chromosphere
and Prominences _To face p. 198_
Plate II. Photograph of the Great Comet of May,
1901 (Taken at the Royal Observatory, Cape
of Good Hope) " 343
Plate III. The Great Comet of September, 1882
(Photographed at the Cape of Good Hope) " 359
Plate IV. Photographs of Swift's Comet, 1892 " 368
Plate V. Photographic and Visual Spectrum of
Nova Aurigæ " 396
Plate VI. Photograph of the Milky Way in Sagittarius " 424
HISTORY OF ASTRONOMY DURING THE NINETEENTH CENTURY
_INTRODUCTION_
We can distinguish three kinds of astronomy, each with a different
origin and history, but all mutually dependent, and composing, in their
fundamental unity, one science. First in order of time came the art of
observing the returns, and measuring the places, of the heavenly bodies.
This was the sole astronomy of the Chinese and Chaldeans; but to it the
vigorous Greek mind added a highly complex geometrical plan of their
movements, for which Copernicus substituted a more harmonious system,
without as yet any idea of a compelling cause. The planets revolved in
circles because it was their nature to do so, just as laudanum sets to
sleep because it possesses a _virtus dormitiva_. This first and oldest
branch is known as "observational," or "practical astronomy." Its
business is to note facts as accurately as possible; and it is
essentially unconcerned with schemes for connecting those facts in a
manner satisfactory to the reason.
The second kind of astronomy was founded by Newton. Its nature is best
indicated by the term "gravitational"; but it is also called
"theoretical astronomy."[1] It is based on the idea of cause; and the
whole of its elaborate structure is reared according to the dictates of
a single law, simple in itself, but the tangled web of whose
consequences can be unravelled only by the subtle agency of an elaborate
calculus.
The third and last division of celestial science may properly be termed
"physical and descriptive astronomy." It seeks to know what the heavenly
bodies are in themselves, leaving the How? and the Wherefore? of their
movements to be otherwise answered. Now, such inquiries became possible
only through the invention of the telescope, so that Galileo was, in
point of fact, their originator. But Herschel first gave them a
prominence which the whole progress of science during the nineteenth
century served to confirm and render more exclusive. Inquisitions begun
with the telescope have been extended and made effective in unhoped-for
directions by the aid of the spectroscope and photographic camera; and a
large part of our attention in the present volume will be occupied with
the brilliant results thus achieved.
The unexpected development of this new physical-celestial science is the
leading fact in recent astronomical history. It was out of the regular
course of events. In the degree in which it has actually occurred it
could certainly not have been foreseen. It was a seizing of the prize by
a competitor who had hardly been thought qualified to enter the lists.
Orthodox astronomers of the old school looked with a certain contempt
upon observers who spent their nights in scrutinising the faces of the
moon and planets rather than in timing their transits, or devoted
daylight energies, not to reductions and computations, but to counting
and measuring spots on the sun. They were regarded as irregular
practitioners, to be tolerated perhaps, but certainly not encouraged.
The advance of astronomy in the eighteenth century ran in general an
even and logical course. The age succeeding Newton's had for its special
task to demonstrate the universal validity, and trace the complex
results, of the law of gravitation. The accomplishment of that task
occupied just one hundred years. It was virtually brought to a close
when Laplace explained to the French Academy, November 19, 1787, the
cause of the moon's accelerated motion. As a mere machine, the solar
system, so far as it was then known, was found to be complete and
intelligible in all its parts; and in the _Mécanique Céleste_ its
mechanical perfections were displayed under a form of majestic unity
which fitly commemorated the successive triumphs of analytical genius
over problems amongst the most arduous ever dealt with by the mind of
man.
Theory, however, demands a practical test. All its data are derived from
observation; and their insecurity becomes less tolerable as it advances
nearer to perfection. Observation, on the other hand, is the pitiless
critic of theory; it detects weak points, and provokes reforms which may
be the beginnings of discovery. Thus, theory and observation mutually
act and react, each alternately taking the lead in the endless race of
improvement.
Now, while in France Lagrange and Laplace were bringing the
gravitational theory of the solar system to completion, work of a very
different kind, yet not less indispensable to the future welfare of
astronomy, was being done in England. The Royal Observatory at Greenwich
is one of the few useful institutions which date their origin from the
reign of Charles II. The leading position which it still occupies in the
science of celestial observation was, for near a century and a half
after its foundation, an exclusive one. Delambre remarked that, had all
other materials of the kind been destroyed, the Greenwich records alone
would suffice for the restoration of astronomy. The establishment was
indeed absolutely without a rival.[2] Systematic observations of sun,
moon, stars, and planets were during the whole of the eighteenth century
made only at Greenwich. Here materials were accumulated for the secure
correction of theory, and here refinements were introduced by which the
exquisite accuracy of modern practice in astronomy was eventually
attained.
The chief promoter of these improvements was James Bradley. Few men have
possessed in an equal degree with him the power of seeing accurately,
and reasoning on what they see. He let nothing pass. The slightest
inconsistency between what appeared and what was to be expected roused
his keenest attention; and he never relaxed his mental grip of a subject
until it had yielded to his persistent inquisition. It was to these
qualities that he owed his discoveries of the aberration of light and
the nutation of the earth's axis. The first was announced in 1729. What
is meant by it is that, owing to the circumstance of light not being
instantaneously transmitted, the heavenly bodies appear shifted from
their true places by an amount depending upon the ratio which the
velocity of light bears to the speed of the earth in its orbit. Because
light travels with enormous rapidity, the shifting is very slight; and
each star returns to its original position at the end of a year.
Bradley's second great discovery was finally ascertained in 1748.
Nutation is a real "nodding" of the terrestrial axis produced by the
dragging of the moon at the terrestrial equatorial protuberance. From it
results an _apparent_ displacement of the stars, each of them describing
a little ellipse about its true or "mean" position, in a period of
nearly nineteen years.
Now, an acquaintance with the fact and the laws of each of these minute
irregularities is vital to the progress of observational astronomy; for
without it the places of the heavenly bodies could never be accurately
known or compared. So that Bradley, by their detection, at once raised
the science to a higher grade of precision. Nor was this the whole of
his work. Appointed Astronomer-Royal in 1742, he executed during the
years 1750-62 a series of observations which formed the real beginning
of exact astronomy. Part of their superiority must, indeed, be
attributed to the co-operation of John Bird, who provided Bradley in
1750 with a measuring instrument of till then unequalled excellence. For
not only was the art of observing in the eighteenth century a peculiarly
English art, but the means of observing were furnished almost
exclusively by British artists. John Dollond, the son of a Spitalfields
weaver, invented the achromatic lens in 1758, removing thereby the chief
obstacle to the development of the powers of refracting telescopes;
James Short, of Edinburgh, was without a rival in the construction of
reflectors; the sectors, quadrants, and circles of Graham, Bird,
Ramsden, and Cary were inimitable by Continental workmanship.
Thus practical and theoretical astronomy advanced on parallel lines in
England and France respectively, the improvement of their several
tools--the telescope and the quadrant on the one side, and the calculus
on the other--keeping pace. The whole future of the science seemed to be
theirs. The cessation of interest through a too speedy attainment of the
perfection towards which each spurred the other, appeared to be the only
danger it held in store for them. When all at once, a rival stood by
their side--not, indeed, menacing their progress, but threatening to
absorb their popularity.
The rise of Herschel was the one conspicuous anomaly in the astronomical
history of the eighteenth century. It proved decisive of the course of
events in the nineteenth. It was unexplained by anything that had gone
before; yet all that came after hinged upon it. It gave a new direction
to effort; it lent a fresh impulse to thought. It opened a channel for
the widespread public interest which was gathering towards astronomical
subjects to flow in.
Much of this interest was due to the occurrence of events calculated to
arrest the attention and excite the wonder of the uninitiated. The
predicted return of Halley's comet in 1759 verified, after an
unprecedented fashion, the computations of astronomers. It deprived such
bodies for ever of their portentous character; it ranked them as
denizens of the solar system. Again, the transits of Venus in 1761 and
1769 were the first occurrences of the kind since the awakening of
science to their consequence. Imposing preparations, journeys to remote
and hardly accessible regions, official expeditions, international
communications, all for the purpose of observing them to the best
advantage, brought their high significance vividly to the public
consciousness; a result aided by the facile pen of Lalande, in rendering
intelligible the means by which these elaborate arrangements were to
issue in an accurate knowledge of the sun's distance. Lastly, Herschel's
discovery of Uranus, March 13, 1781, had the surprising effect of utter
novelty. Since the human race had become acquainted with the company of
the planets, no addition had been made to their number. The event thus
broke with immemorial traditions, and seemed to show astronomy as still
young and full of unlooked-for possibilities.
Further popularity accrued to the science from the sequel of a career so
strikingly opened. Herschel's huge telescopes, his detection by their
means of two Saturnian and as many Uranian moons, his piercing scrutiny
of the sun, picturesque theory of its constitution, and sagacious
indication of the route pursued by it through space; his discovery of
stellar revolving systems, his bold soundings of the universe, his
grandiose ideas, and the elevated yet simple language in which they were
conveyed--formed a combination powerfully effective to those least
susceptible of new impressions. Nor was the evoked enthusiasm limited to
the British Isles. In Germany, Schröter followed--_longo intervallo_--in
Herschel's track. Von Zach set on foot from Gotha that general
communication of ideas which gives life to a forward movement. Bode
wrote much and well for unlearned readers. Lalande, by his popular
lectures and treatises, helped to form an audience which Laplace himself
did not disdain to address in the _Exposition du Système du Monde_.
This great accession of public interest gave the impulse to the
extraordinarily rapid progress of astronomy in the nineteenth century.
Official patronage combined with individual zeal sufficed for the elder
branches of the science. A few well-endowed institutions could
accumulate the materials needed by a few isolated thinkers for the
construction of theories of wonderful beauty and elaboration, yet
precluded, by their abstract nature, from winning general applause. But
the new physical astronomy depends for its prosperity upon the favour of
the multitude whom its striking results are well fitted to attract. It
is, in a special manner, the science of amateurs. It welcomes the most
unpretending co-operation. There is no one "with a true eye and a
faithful hand" but can do good work in watching the heavens. And not
unfrequently, prizes of discovery which the most perfect appliances
failed to grasp, have fallen to the share of ignorant or ill-provided
assiduity.
Observers, accordingly, have multiplied; observatories have been founded
in all parts of the world; associations have been constituted for mutual
help and counsel. A formal astronomical congress met in 1789 at
Gotha--then, under Duke Ernest II. and Von Zach, the focus of German
astronomy--and instituted a combined search for the planet suspected to
revolve undiscovered between the orbits of Mars and Jupiter. The
Astronomical Society of London was established in 1820, and the similar
German institution in 1863. Both have been highly influential in
promoting the interests, local and general, of the science they are
devoted to forward; while functions corresponding to theirs have been
discharged elsewhere by older or less specially constituted bodies, and
new ones of a more popular character are springing up on all sides.
Modern facilities of communication have helped to impress more deeply
upon modern astronomy its associative character. The electric telegraph
gives a certain ubiquity which is invaluable to an observer of the
skies. With the help of a wire, a battery, and a code of signals, he
sees whatever is visible from any portion of our globe, depending,
however, upon other eyes than his own, and so entering as a unit into a
widespread organisation of intelligence. The press, again, has been a
potent agent of co-operation. It has mainly contributed to unite
astronomers all over the world into a body animated by the single aim of
collecting "particulars" in their special branch for what Bacon termed a
History of Nature, eventually to be interpreted according to the
sagacious insight of some one among them gifted above his fellows. The
first really effective astronomical periodical was the _Monatliche
Correspondenz_, started by Von Zach in the year 1800. It was followed in
1822 by the _Astronomische Nachrichten_, later by the _Memoirs_ and
_Monthly Notices_ of the Astronomical Society, and by the host of varied
publications which now, in every civilised country, communicate the
discoveries made in astronomy to divers classes of readers, and so
incalculably quicken the current of its onward flow.
Public favour brings in its train material resources. It is represented
by individual enterprise, and finds expression in an ample liberality.
The first regular observatory in the Southern Hemisphere was founded at
Paramatta by Sir Thomas Makdougall Brisbane in 1821. The Royal
Observatory at the Cape of Good Hope was completed in 1829. Similar
establishments were set to work by the East India Company at Madras,
Bombay, and St. Helena, during the first third of the nineteenth
century. The organisation of astronomy in the United States of America
was due to a strong wave of popular enthusiasm. In 1825 John Quincy
Adams vainly urged upon Congress the foundation of a National
Observatory; but in 1843 the lectures on celestial phenomena of Ormsby
MacKnight Mitchel stirred an impressionable audience to the pitch of
providing him with the means of erecting at Cincinnati the first
astronomical establishment worthy the name in that great country. On the
1st of January, 1882, no less than one hundred and forty-four were
active within its boundaries.
The apparition of the great comet of 1843 gave an additional fillip to
the movement. To the excitement caused by it the Harvard College
Observatory--called the "American Pulkowa"--directly owed its origin;
and the example was not ineffective elsewhere. The United States Naval
Observatory was built in 1844, Lieutenant Maury being its first
Director. Corporations, universities, municipalities, vied with each
other in the creation of such institutions; private subscriptions poured
in; emissaries were sent to Europe to purchase instruments and to
procure instruction in their use. In a few years the young Republic was,
in point of astronomical efficiency, at least on a level with countries
where the science had been fostered since the dawn of civilisation.
A vast widening of the scope of astronomy has accompanied, and in part
occasioned, the great extension of its area of cultivation which our age
has witnessed. In the last century its purview was a comparatively
narrow one. Problems lying beyond the range of the solar system were
almost unheeded, because they seemed inscrutable. Herschel first showed
the sidereal universe as accessible to investigation, and thereby
offered to science new worlds--majestic, manifold, "infinitely infinite"
to our apprehension in number, variety, and extent--for future conquest.
Their gradual appropriation has absorbed, and will long continue to
absorb, the powers which it has served to develop.
But this is not the only direction in which astronomy has enlarged, or
rather has levelled, its boundaries. The unification of the physical
sciences is perhaps the greatest intellectual feat of recent times. The
process has included astronomy; so that, like Bacon, she may now be said
to have "taken all knowledge" (of that kind) "for her province." In
return, she proffers potent aid for its increase. Every comet that
approaches the sun is the scene of experiments in the electrical
illumination of rarefied matter, performed on a huge scale for our
benefit. The sun, stars, and nebulæ form so many celestial laboratories,
where the nature and mutual relations of the chemical "elements" may be
tried by more stringent tests than sublunary conditions afford. The laws
of terrestrial magnetism can be completely investigated only with the
aid of a concurrent study of the face of the sun. The solar spectrum
will perhaps one day, by its recurrent modifications, tell us something
of impending droughts, famines, and cyclones.
Astronomy generalises the results of the other sciences. She exhibits
the laws of Nature working over a wider area, and under more varied
conditions, than ordinary experience presents. Ordinary experience, on
the other hand, has become indispensable to her progress. She takes in
at one view the indefinitely great and the indefinitely little. The
mutual revolutions of the stellar multitude during tracts of time which
seem to lengthen out to eternity as the mind attempts to traverse them,
she does not admit to be beyond her ken; nor is she indifferent to the
constitution of the minutest atom of matter that thrills the ether into
light. How she entered upon this vastly expanded inheritance, and how,
so far, she has dealt with it, is attempted to be set forth in the
ensuing chapters.
FOOTNOTES:
[Footnote 1: The denomination "physical astronomy," first used by
Kepler, and long appropriated to this branch of the science, has of late
been otherwise applied.]
[Footnote 2: _Histoire de l'Astronomie au xviii^e Siècle_, p. 267.]
PART I
PROGRESS OF ASTRONOMY DURING THE FIRST HALF OF THE NINETEENTH CENTURY
CHAPTER I
_FOUNDATION OF SIDEREAL ASTRONOMY_
Until nearly a hundred years ago the stars were regarded by practical
astronomers mainly as a number of convenient fixed points by which the
motions of the various members of the solar system could be determined
and compared. Their recognised function, in fact, was that of milestones
on the great celestial highway traversed by the planets, as well as on
the byways of space occasionally pursued by comets. Not that curiosity
as to their nature, and even conjecture as to their origin, were at any
period absent. Both were from time to time powerfully stimulated by the
appearance of startling novelties in a region described by philosophers
as "incorruptible," or exempt from change. The catalogue of Hipparchus
probably, and certainly that of Tycho Brahe, some seventeen centuries
later, owed each its origin to the temporary blaze of a new star. The
general aspect of the skies was thus (however imperfectly) recorded from
age to age, and with improved appliances the enumeration was rendered
more and more accurate and complete; but the secrets of the stellar
sphere remained inviolate.
In a qualified though very real sense, Sir William Herschel may be
called the Founder of Sidereal Astronomy. Before his time some curious
facts had been noted, and some ingenious speculations hazarded,
regarding the condition of the stars, but not even the rudiments of
systematic knowledge had been acquired. The facts ascertained can be
summed up in a very few sentences.
Giordano Bruno was the first to set the suns of space in motion; but in
imagination only. His daring surmise was, however, confirmed in 1718,
when Halley announced[3] that Sirius, Aldebaran, Betelgeux, and Arcturus
had unmistakably shifted their quarters in the sky since Ptolemy
assigned their places in his catalogue. A similar conclusion was reached
by J. Cassini in 1738, from a comparison of his own observations with
those made at Cayenne by Richer in 1672; and Tobias Mayer drew up in
1756 a list showing the direction and amount of about fifty-seven proper
motions,[4] founded on star-places determined by Olaus Römer fifty years
previously. Thus the stars were no longer regarded as "fixed," but the
question remained whether the movements perceived were real or only
apparent; and this it was not yet found possible to answer. Already, in
the previous century, the ingenious Robert Hooke had suggested an
"alteration of the very system of the sun,"[5] to account for certain
suspected changes in stellar positions; Bradley in 1748, and Lambert in
1761, pointed out that such apparent displacements (by that time well
ascertained) were in all probability a combined effect of motions both
of sun and stars; and Mayer actually attempted the analysis, but without
result.
On the 13th of August, 1596, David Fabricius, an unprofessional
astronomer in East Friesland, saw in the neck of the Whale a star of the
third magnitude, which by October had disappeared. It was, nevertheless,
visible in 1603, when Bayer marked it in his catalogue with the Greek
letter Omicron, and was watched, in 1638-39, through its phases of
brightening and apparent extinction by a Dutch professor named
Holwarda.[6] From Hevelius this first-known periodical star received the
name of "Mira," or the Wonderful, and Boulliaud in 1667 fixed the length
of its cycle of change at 334 days. It was not a solitary instance. A
star in the Swan was perceived by Janson in 1600 to show fluctuations of
light, and Montanari found in 1669 that Algol in Perseus shared the same
peculiarity to a marked degree. Altogether the class embraced in 1782
half-a-dozen members. When it is added that a few star-couples had been
noted in singularly, but it was supposed accidentally, close
juxtaposition, and that the failure of repeated attempts to measure
stellar parallaxes pointed to distances _at least_ 400,000 times that of
the earth from the sun,[7] the picture of sidereal science, when the
last quarter of the eighteenth century began, is practically complete.
It included three items of information: that the stars have motions,
real or apparent; that they are immeasurably remote; and that a few
shine with a periodically variable light. Nor were these scantily
collected facts ordered into any promise of further development. They
lay at once isolated and confused before the inquirer. They needed to be
both multiplied and marshalled, and it seemed as if centuries of patient
toil must elapse before any reliable conclusions could be derived from
them. The sidereal world was thus the recognised domain of far-reaching
speculations, which remained wholly uncramped by systematic research
until Herschel entered upon his career as an observer of the heavens.
The greatest of modern astronomers was born at Hanover, November 15,
1738. He was the fourth child of Isaac Herschel, a hautboy-player in the
band of the Hanoverian Guard, and was early trained to follow his
father's profession. On the termination, however, of the disastrous
campaign of 1757, his parents removed him from the regiment, there is
reason to believe, in a somewhat unceremonious manner. Technically,
indeed, he incurred the penalties of desertion, remitted--according to
the Duke of Sussex's statement to Sir George Airy--by a formal pardon
handed to him personally by George III. on his presentation in 1782.[8]
At the age of nineteen, then, his military service having lasted four
years, he came to England to seek his fortune. Of the life of struggle
and privation which ensued little is known beyond the circumstances that
in 1760 he was engaged in training the regimental band of the Durham
Militia, and that in 1765 he was appointed organist at Halifax. In the
following year he removed to Bath as oboist in Linley's orchestra, and
in October 1767 was promoted to the post of organist in the Octagon
Chapel. The tide of prosperity now began to flow for him. The most
brilliant and modish society in England was at that time to be met at
Bath, and the young Hanoverian quickly found himself a favourite and the
fashion in it. Engagements multiplied upon him. He became director of
the public concerts; he conducted oratorios, engaged singers, organised
rehearsals, composed anthems, chants, choral services, besides
undertaking private tuitions, at times amounting to thirty-five or even
thirty-eight lessons a week. He in fact personified the musical activity
of a place then eminently and energetically musical.
But these multifarious avocations did not take up the whole of his
thoughts. His education, notwithstanding the poverty of his family, had
not been neglected, and he had always greedily assimilated every kind of
knowledge that came in his way. Now that he was a busy and a prosperous
man, it might have been expected that he would run on in the deep
professional groove laid down for him. On the contrary, his passion for
learning seemed to increase with the diminution of the time available
for its gratification. He studied Italian, Greek, mathematics;
Maclaurin's Fluxions served to "unbend his mind"; Smith's Harmonics and
Optics and Ferguson's Astronomy were the nightly companions of his
pillow. What he read stimulated without satisfying his intellect. He
desired not only to know, but to discover. In 1772 he hired a small
telescope, and through it caught a preliminary glimpse of the rich and
varied fields in which for so many years he was to expatiate.
Henceforward the purpose of his life was fixed: it was to obtain "a
knowledge of the construction of the heavens";[9] and this sublime
ambition he cherished to the end.
A more powerful instrument was the first desideratum; and here his
mechanical genius came to his aid. Having purchased the apparatus of a
Quaker optician, he set about the manufacture of specula with a zeal
which seemed to anticipate the wonders they were to disclose to him. It
was not until fifteen years later that his grinding and polishing
machines were invented, so the work had at that time to be entirely done
by hand. During this tedious and laborious process (which could not be
interrupted without injury, and lasted on one occasion sixteen hours),
his strength was supported by morsels of food put into his mouth by his
sister,[10] and his mind amused by her reading aloud to him the Arabian
Nights, Don Quixote, or other light works. At length, after repeated
failures, he found himself provided with a reflecting telescope--a
5-1/2-foot Gregorian--of his own construction. A copy of his first
observation with it, on the great Nebula in Orion--an object of
continual amazement and assiduous inquiry to him--is preserved by the
Royal Society. It bears the date March 4, 1774.[11]
In the following year he executed his first "review of the heavens,"
memorable chiefly as an evidence of the grand and novel conceptions
which already inspired him, and of the enthusiasm with which he
delivered himself up to their guidance. Overwhelmed with professional
engagements, he still contrived to snatch some moments for the stars;
and between the acts at the theatre was often seen running from the
harpsichord to his telescope, no doubt with that "uncommon precipitancy
which accompanied all his actions."[12] He now rapidly increased the
power and perfection of his telescopes. Mirrors of seven, ten, even
twenty feet focal length, were successively completed, and unprecedented
magnifying powers employed. His energy was unceasing, his perseverance
indomitable. In the course of twenty-one years no less than 430
parabolic specula left his hands. He had entered upon his forty-second
year when he sent his first paper to the _Philosophical Transactions_;
yet during the ensuing thirty-nine years his contributions--many of them
elaborate treatises--numbered sixty-nine, forming a series of
extraordinary importance to the history of astronomy. As a mere explorer
of the heavens his labours were prodigious. He discovered 2,500 nebulæ,
806 double stars, passed the whole firmament in review four several
times, counted the stars in 3,400 "gauge-fields," and executed a
photometric classification of the principal stars, founded on an
elaborate (and the first systematically conducted) investigation of
their relative brightness. He was as careful and patient as he was
rapid; spared no time and omitted no precaution to secure accuracy in
his observations; yet in one night he would examine, singly and
attentively, up to 400 separate objects.
The discovery of Uranus was a mere incident of the scheme he had marked
out for himself--a fruit, gathered as it were by the way. It formed,
nevertheless, the turning-point in his career. From a star-gazing
musician he was at once transformed into an eminent astronomer; he was
relieved from the drudgery of a toilsome profession, and installed as
Royal Astronomer, with a modest salary of £200 a year; funds were
provided for the construction of the forty-foot reflector, from the
great space-penetrating power of which he expected unheard-of
revelations; in fine, his future work was not only rendered possible,
but it was stamped as authoritative.[13] On Whit-Sunday 1782, William
and Caroline Herschel played and sang in public for the last time in St.
Margaret's Chapel, Bath; in August of the same year the household was
moved to Datchet, near Windsor, and on April 3, 1786, to Slough. Here
happiness and honours crowded on the fortunate discoverer. In 1788 he
married Mary, only child of James Baldwin, a merchant of the city of
London, and widow of Mr. John Pitt--a lady whose domestic virtues were
enhanced by the possession of a large jointure. The fruit of their union
was one son, of whose work--the worthy sequel of his father's--we shall
have to speak further on. Herschel was created a Knight of the
Hanoverian Guelphic Order in 1816, and in 1821 he became the first
President of the Royal Astronomical Society, his son being its first
Foreign Secretary. But his health had now for some years been failing,
and on August 25, 1822, he died at Slough, in the eighty-fourth year of
his age, and was buried in Upton churchyard.
His epitaph claims for him the lofty praise of having "burst the
barriers of heaven." Let us see in what sense this is true.
The first to form any definite idea as to the constitution of the
stellar system was Thomas Wright, the son of a carpenter living at
Byer's Green, near Durham. With him originated what has been called the
"Grindstone Theory" of the universe, which regarded the Milky Way as the
projection on the sphere of a stratum or disc of stars (our sun
occupying a position near the centre), similar in magnitude and
distribution to the lucid orbs of the constellations.[14] He was
followed by Kant,[15] who transcended the views of his predecessor by
assigning to nebulæ the position they long continued to occupy, rather
on imaginative than scientific grounds, of "island universes," external
to, and co-equal with, the Galaxy. Johann Heinrich Lambert,[16] a
tailor's apprentice from Mühlhausen, followed, but independently. The
conceptions of this remarkable man were grandiose, his intuitions bold,
his views on some points a singular anticipation of subsequent
discoveries. The sidereal world presented itself to him as a hierarchy
of systems, starting from the planetary scheme, rising to throngs of
suns within the circuit of the Milky Way--the "ecliptic of the stars,"
as he phrased it--expanding to include groups of many Milky Ways; these
again combining to form the unit of a higher order of assemblage, and so
onwards and upwards until the mind reels and sinks before the immensity
of the contemplated creations.
"Thus everything revolves--the earth round the sun; the sun round the
centre of his system; this system round a centre common to it with other
systems; this group, this assemblage of systems, round a centre which is
common to it with other groups of the same kind; and where shall we have
done?"[17]
The stupendous problem thus speculatively attempted, Herschel undertook
to grapple with experimentally. The upshot of this memorable inquiry was
the inclusion, for the first time, within the sphere of human knowledge,
of a connected body of facts, and inferences from facts, regarding the
sidereal universe; in other words, the foundation of what may properly
be called a science of the stars.
Tobias Mayer had illustrated the perspective effects which must ensue in
the stellar sphere from a translation of the solar system, by comparing
them to the separating in front and closing up behind of trees in a
forest to the eye of an advancing spectator;[18] but the appearances
which he thus correctly described he was unable to detect. By a more
searching analysis of a smaller collection of proper motions, Herschel
succeeded in rendering apparent the very consequences foreseen by Mayer.
He showed, for example, that Arcturus and Vega did, in fact, appear to
recede from, and Sirius and Aldebaran to approach, each other by very
minute amounts; and, with a striking effort of divinatory genius, placed
the "apex," or point of direction of the sun's motion, close to the star
Lambda in the constellation Hercules,[19] within a few degrees of
the spot indicated by later and indefinitely more refined methods of
research. He resumed the subject in 1805,[20] but though employing a
more rigorous method, was scarcely so happy in his result. In 1806,[21]
he made a preliminary attempt to ascertain the speed of the sun's
journey, fixing it, by doubtless much too low an estimate, at about
three miles a second. Yet the validity of his general conclusion as to
the line of solar travel, though long doubted, has been triumphantly
confirmed. The question as to the "secular parallax" of the fixed stars
was in effect answered.
With their _annual_ parallax, however, the case was very different. The
search for it had already led Bradley to the important discoveries of
the aberration of light and the nutation of the earth's axis; it was now
about to lead Herschel to a discovery of a different, but even more
elevated character. Yet in neither case was the object primarily sought
attained.
From the very first promulgation of the Copernician theory the seeming
immobility of the stars had been urged as an argument against its truth;
for if the earth really travelled in a vast orbit round the sun, objects
in surrounding space should appear to change their positions, unless
their distances were on a scale which, to the narrow ideas of the
universe then prevailing, seemed altogether extravagant.[22] The
existence of such apparent or "parallactic" displacements was
accordingly regarded as the touchstone of the new views, and their
detection became an object of earnest desire to those interested in
maintaining them. Copernicus himself made the attempt; but with his
"Triquetrum," a jointed wooden rule with the divisions marked in ink,
constructed by himself,[23] he was hardly able to measure angles of ten
minutes, far less fractions of a second. Galileo, a more impassioned
defender of the system, strained his ears, as it were, from Arcetri, in
his blind and sorrowful old age, for news of a discovery which two more
centuries had still to wait for. Hooke believed he had found a parallax
for the bright star in the Head of the Dragon; but was deceived. Bradley
convinced himself that such effects were too minute for his instruments
to measure. Herschel made a fresh attempt by a practically untried
method.
It is a matter of daily experience that two objects situated at
different distances seem to a beholder in motion to move relatively to
each other. This principle Galileo, in the third of his Dialogues on the
Systems of the World,[24] proposed to employ for the determination of
stellar parallax; for two stars, lying apparently close together, but in
reality separated by a great gulf of space, must shift their mutual
positions when observed from opposite points of the earth's orbit; or
rather, the remoter forms a virtually fixed point, to which the
movements of the other can be conveniently referred. By this means
complications were abolished more numerous and perplexing than Galileo
himself was aware of, and the problem was reduced to one of simple
micrometrical measurement. The "double-star method" was also suggested
by James Gregory in 1675, and again by Wallis in 1693;[25] Huygens
first, and afterwards Dr. Long of Cambridge (about 1750), made futile
experiments with it; and it eventually led, in the hands of Bessel, to
the successful determination of the parallax of 61 Cygni.
Its advantages were not lost upon Herschel. His attempt to assign
definite distances to the nearest stars was no isolated effort, but part
of the settled plan upon which his observations were conducted. He
proposed to sound the heavens, and the first requisite was a knowledge
of the length of his sounding-line. Thus it came about that his special
attention was early directed to double stars.
"I resolved," he writes,[26] "to examine every star in the heavens with
the utmost attention and a very high power, that I might collect such
materials for this research as would enable me to fix my observations
upon those that would best answer my end. The subject has already proved
so extensive, and still promises so rich a harvest to those who are
inclined to be diligent in the pursuit, that I cannot help inviting
every lover of astronomy to join with me in observations that must
inevitably lead to new discoveries."
The first result of these inquiries was a classed catalogue of 269
double stars presented to the Royal Society in 1782, followed, after
three years, by an additional list of 434. In both these collections the
distances separating the individuals of each pair were carefully
measured, and (with a few exceptions) the angles made with the
hour-circle by the lines joining their centres (technically called
"angles of position") were determined with the aid of a "revolving-wire
micrometer," specially devised for the purpose. Moreover, an important
novelty was introduced by the observation of the various colours visible
in the star-couples, the singular and vivid contrasts of which were now
for the first time described.
Double stars were at that time supposed to be a purely optical
phenomenon. Their components, it was thought, while in reality
indefinitely remote from each other, were brought into fortuitous
contiguity by the chance of lying nearly in the same line of sight from
the earth. Yet Bradley had noticed a change of 30°, between 1718 and
1759, in the position-angle of the two stars forming Castor, and was
thus within a hair's breadth of the discovery of their physical
connection.[27] While the Rev. John Michell, arguing by the doctrine of
probabilities, wrote as follows in 1767:--"It is highly probable in
particular, and next to a certainty in general, that such double stars
as appear to consist of two or more stars placed very near together, do
really consist of stars placed near together, and under the influence of
some general law."[28] And in 1784:[29] "It is not improbable that a few
years may inform us that some of the great number of double, triple
stars, etc., which have been observed by Mr. Herschel, are systems of
bodies revolving about each other."
This remarkable speculative anticipation had a practical counterpart in
Germany. Father Christian Mayer, a Jesuit astronomer at Mannheim, set
himself, in January 1776, to collect examples of stellar pairs, and
shortly after published the supposed discovery of "satellites" to many
of the principal stars.[30] But his observations were neither exact nor
prolonged enough to lead to useful results in such an inquiry. His
disclosures were derided; his planet-stars treated as results of
hallucination. _On n'a point cru à des choses aussi extraordinaires_,
wrote Lalande[31] within one year of a better-grounded announcement to
the same effect.
Herschel at first shared the general opinion as to the merely optical
connection of double stars. Of this the purpose for which he made his
collection is in itself sufficient evidence, since what may be called
the _differential_ method of parallaxes depends, as we have seen, for
its efficacy upon disparity of distance. It was "much too soon," he
declared in 1782,[32] "to form any theories of small stars revolving
round large ones;" while in the year following,[33] he remarked that the
identical proper motions of the two stars forming, to the naked eye, the
single bright orb of Castor could only be explained as both equally due
to the "systematic parallax" caused by the sun's movement in space.
Plainly showing that the notion of a physical tie, compelling the two
bodies to travel together, had not as yet entered into his speculations.
But he was eminently open to conviction, and had, moreover, by
observations unparalleled in amount as well as in kind, prepared ample
materials for convincing himself and others. In 1802 he was able to
announce the fact of his discovery, and in the two ensuing years, to lay
in detail before the Royal Society proofs, gathered from the labours of
a quarter of a century, of orbital revolution in the case of as many as
fifty double stars, henceforth, he declared, to be held as real binary
combinations, "intimately held together by the bond of mutual
attraction."[34] The fortunate preservation in Dr. Maskelyne's note-book
of a remark made by Bradley about 1759, to the effect that the line
joining the components of Castor was an exact prolongation of that
joining Castor with Pollux, added eighteen years to the time during
which the pair were under scrutiny, and confirmed the evidence of change
afforded by more recent observations. Approximate periods were fixed for
many of the revolving suns--for Castor 342 years; for Gamma Leonis,
1200, Delta Serpentis, 375, Eta Bootis, 1681 years; Eta Lyræ
was noted as a "double-double-star," a change of relative
situation having been detected in each of the two pairs composing the
group; and the occultation was described of one star by another in the
course of their mutual revolutions, as exemplified in 1795 by the
rapidly circulating system of Zeta Herculis.
Thus, by the sagacity and perseverance of a single observer, a firm
basis was at last provided upon which to raise the edifice of sidereal
science. The analogy long presumed to exist between the mighty star of
our system and the bright points of light spangling the firmament was
shown to be no fiction of the imagination, but a physical reality; the
fundamental quality of attractive power was proved to be common to
matter so far as the telescope was capable of exploring, and law,
subordination, and regularity to give testimony of supreme and
intelligent design no less in those limitless regions of space than in
our narrow terrestrial home. The discovery was emphatically (in Arago's
phrase) "one with a future," since it introduced the element of precise
knowledge where more or less probable conjecture had previously held
almost undivided sway; and precise knowledge tends to propagate itself
and advance from point to point.
We have now to speak of Herschel's pioneering work in the skies. To
explore with line and plummet the shining zone of the Milky Way, to
delineate its form, measure its dimensions, and search out the
intricacies of its construction, was the primary task of his life, which
he never lost sight of, and to which all his other investigations were
subordinate. He was absolutely alone in this bold endeavour. Unaided, he
had to devise methods, accumulate materials, and sift out results. Yet
it may safely be asserted that all the knowledge we possess on this
sublime subject was prepared, and the greater part of it anticipated, by
him.
The ingenious method of "star-gauging," and its issue in the delineation
of the sidereal system as an irregular stratum of evenly-scattered suns,
is the best-known part of his work. But it was, in truth, only a first
rude approximation, the principle of which maintained its credit in the
literature of astronomy a full half-century after its abandonment by its
author. This principle was the general equality of star distribution. If
equal portions of space really held equal numbers of stars, it is
obvious that the number of stars visible in any particular direction
would be strictly proportional to the range of the system in that
direction, apparent accumulation being produced by real extent. The
process of "gauging the heavens," accordingly, consisted in counting the
stars in successive telescopic fields, and calculating thence the depths
of space necessary to contain them. The result of 3,400 such operations
was the plan of the Galaxy familiar to every reader of an astronomical
text-book. Widely-varying evidence was, as might have been expected,
derived from an examination of different portions of the sky. Some
fields of view were almost blank, while others (in or near the Milky
Way) blazed with the radiance of many hundred stars compressed into an
area about one-fourth that of the full-moon. In the most crowded parts
116,000 were stated to have been passed in review within a quarter of an
hour. Here the "length of his sounding-line" was estimated by Herschel
at about 497 times the distance of Sirius--in other words, the bounding
orb, or farthest sun of the system in that direction, so far as could be
seen with the 20-foot reflector, was thus inconceivably remote. But
since the distance of Sirius, no less than of every other fixed star,
was as yet an unknown quantity, the dimensions inferred for the Galaxy
were of course purely relative; a knowledge of its form and structure
might (admitting the truth of the fundamental hypothesis) be obtained,
but its real or absolute size remained altogether undetermined.
Even as early as 1785, however, Herschel perceived traces of a tendency
which completely invalidated the supposition of any approach to an
average uniformity of distribution. This was the action of what he
called a "clustering power" in the Milky Way. "Many gathering
clusters"[35] were already discernible to him even while he endeavoured
to obtain a "true _mean_ result" on the assumption that each star in
space was separated from its neighbours as widely as the sun from
Sirius. "It appears," he wrote in 1789, "that the heavens consist of
regions where suns are gathered into separate systems"; and in certain
assemblages he was able to trace "a course or tide of stars setting
towards a centre," denoting, not doubtfully, the presence of attractive
forces.[36] Thirteen years later, he described our sun and his
constellated companions as surrounded by "a magnificent collection of
innumerable stars, called the Milky Way, which must occasion a very
powerful balance of opposite attractions to hold the intermediate stars
at rest. For though our sun, and all the stars we see, may truly be said
to be in the plane of the Milky Way, yet I am now convinced, by a long
inspection and continued examination of it, that the Milky Way itself
consists of stars very differently scattered from those which are
immediately about us." "This immense aggregation," he added, "is by no
means uniform. Its component stars show evident signs of clustering
together into many separate allotments."[37]
The following sentences, written in 1811, contain a definite
retractation of the view frequently attributed to him:--
"I must freely confess," he says, "that by continuing my sweeps of the
heavens my opinion of the arrangement of the stars and their magnitudes,
and of some other particulars, has undergone a gradual change; and
indeed, when the novelty of the subject is considered, we cannot be
surprised that many things formerly taken for granted should on
examination prove to be different from what they were generally but
incautiously supposed to be. For instance, an equal scattering of the
stars may be admitted in certain calculations; but when we examine the
Milky Way, or the closely compressed clusters of stars of which my
catalogues have recorded so many instances, this supposed equality of
scattering must be given up."[38]
Another assumption, the fallacy of which he had not the means of
detecting since become available, was retained by him to the end of his
life. It was that the brightness of a star afforded an approximate
measure of its distance. Upon this principle he founded in 1817 his
method of "limiting apertures,"[39] by which two stars, brought into
view in two precisely similar telescopes, were "equalised" by covering a
certain portion of the object-glass collecting the more brilliant rays.
The distances of the orbs compared were then taken to be in the ratio of
the reduced to the original apertures of the instruments with which they
were examined. If indeed the absolute lustre of each were the same, the
result might be accepted with confidence; but since we have no warrant
for assuming a "standard star" to facilitate our computations, but much
reason to suppose an indefinite range, not only of size but of intrinsic
brilliancy, in the suns of our firmament, conclusions drawn from such a
comparison are entirely worthless.
In another branch of sidereal science besides that of stellar
aggregation, Herschel may justly be styled a pioneer. He was the first
to bestow serious study on the enigmatical objects known as "nebulæ."
The history of the acquaintance of our race with them is comparatively
short. The only one recognised before the invention of the telescope was
that in the girdle of Andromeda, certainly familiar in the middle of the
tenth century to the Persian astronomer Abdurrahman Al-Sûfi; and marked
with dots on Spanish and Dutch constellation-charts of the fourteenth
and fifteenth centuries.[40] Yet so little was it noticed that it might
practically be said--as far as Europe is concerned--to have been
discovered in 1612 by Simon Marius (Mayer of Genzenhausen), who aptly
described its appearance as that of a "candle shining through horn." The
first mention of the great Orion nebula is by a Swiss Jesuit named
Cysatus, who succeeded Father Scheiner in the chair of mathematics at
Ingolstadt. He used it, apparently without any suspicion of its novelty,
as a term of comparison for the comet of December 1618.[41] A novelty,
nevertheless, to astronomers it still remained in 1656, when Huygens
discerned, "as it were, an hiatus in the sky, affording a glimpse of a
more luminous region beyond."[42] Halley in 1716 knew of six nebulæ,
which he believed to be composed of a "lucid medium" diffused through
the ether of space.[43] He appears, however, to have been unacquainted
with some previously noticed by Hevelius. Lacaille brought back with him
from the Cape a list of forty-two--the first-fruits of observation in
Southern skies--arranged in three numerically equal classes;[44] and
Messier (nicknamed by Louis XV. the "ferret of comets"), finding such
objects a source of extreme perplexity in the pursuit of his chosen
game, attempted to eliminate by methodising them, and drew up a
catalogue comprising, in 1781, 103 entries.[45]
These preliminary attempts shrank into insignificance when Herschel
began to "sweep the heavens" with his giant telescopes. In 1786 he
presented to the Royal Society a descriptive catalogue of 1,000 nebulæ
and clusters, followed, three years later, by a second of as many more;
to which he added in 1802 a further gleaning of 500. On the subject of
their nature his views underwent a remarkable change. Finding that his
potent instruments resolved into stars many nebulous patches in which no
signs of such a structure had previously been discernible, he naturally
concluded that "resolvability" was merely a question of distance and
telescopic power. He was (as he said himself) led on by almost
imperceptible degrees from evident clusters, such as the Pleiades, to
spots without a trace of stellar formation, the gradations being so well
connected as to leave no doubt that all these phenomena were equally
stellar. The singular variety of their appearance was thus described by
him:--
"I have seen," he says, "double and treble nebulæ variously arranged;
large ones with small, seeming attendants; narrow, but much extended
lucid nebulæ or bright dashes; some of the shape of a fan, resembling an
electric brush, issuing from a lucid point; others of the cometic shape,
with a seeming nucleus in the centre, or like cloudy stars surrounded
with a nebulous atmosphere; a different sort, again, contain a
nebulosity of the milky kind, like that wonderful, inexplicable
phenomenon about Theta Orionis; while others shine with a fainter,
mottled kind of light, which denotes their being resolvable into
stars."[46]
"These curious objects" he considered to be "no less than whole sidereal
systems,"[47] some of which might "well outvie our Milky Way in
grandeur." He admitted, however, a wide diversity in condition as well
as compass. The system to which our sun belongs he described as "a very
extensive branching congeries of many millions of stars, which probably
owes its origin to many remarkably large as well as pretty closely
scattered small stars, that may have drawn together the rest."[48] But
the continued action of this same "clustering power" would, he supposed,
eventually lead to the breaking-up of the original majestic Galaxy into
two or three hundred separate groups, already visibly gathering. Such
minor nebulæ, due to the "decay" of other "branching nebulæ" similar to
our own, he recognised by the score, lying, as it were, stratified in
certain quarters of the sky. "One of these nebulous beds," he informs
us, "is so rich that in passing through a section of it, in the time of
only thirty-six minutes, I detected no less than thirty-one nebulæ, all
distinctly visible upon a fine blue sky." The stratum of Coma Berenices
he judged to be the nearest to our system of such layers; nor did the
marked aggregation of nebulæ towards both poles of the circle of the
Milky Way escape his notice.
By a continuation of the same process of reasoning, he was enabled (as
he thought) to trace the life-history of nebulæ from a primitive loose
and extended formation, through clusters of gradually increasing
compression, down to the kind named by him "Planetary" because of the
defined and uniform discs which they present. These he regarded as "very
aged, and drawing on towards a period of change or dissolution."[49]
"This method of viewing the heavens," he concluded, "seems to throw them
into a new kind of light. They now are seen to resemble a luxuriant
garden which contains the greatest variety of productions in different
flourishing beds; and one advantage we may at least reap from it is,
that we can, as it were, extend the range of our experience to an
immense duration. For, to continue the simile which I have borrowed from
the vegetable kingdom, is it not almost the same thing whether we live
successively to witness the germination, blooming, foliage, fecundity,
fading, withering, and corruption of a plant, or whether a vast number
of specimens, selected from every stage through which the plant passes
in the course of its existence, be brought at once to our view?"[50]
But already this supposed continuity was broken. After mature
deliberation on the phenomena presented by nebulous stars, Herschel was
induced, in 1791, to modify essentially his original opinion.
"When I pursued these researches," he says, "I was in the situation of a
natural philosopher who follows the various species of animals and
insects from the height of their perfection down to the lowest ebb of
life; when, arriving at the vegetable kingdom, he can scarcely point out
to us the precise boundary where the animal ceases and the plant begins;
and may even go so far as to suspect them not to be essentially
different. But, recollecting himself, he compares, for instance, one of
the human species to a tree, and all doubt upon the subject vanishes
before him. In the same manner we pass through gentle steps from a
coarse cluster of stars, such as the Pleiades ... till we find ourselves
brought to an object such as the nebula in Orion, where we are still
inclined to remain in the once adopted idea of stars exceedingly remote
and inconceivably crowded, as being the occasion of that remarkable
appearance. It seems, therefore, to require a more dissimilar object to
set us right again. A glance like that of the naturalist, who casts his
eye from the perfect animal to the perfect vegetable, is wanting to
remove the veil from the mind of the astronomer. The object I have
mentioned above is the phenomenon that was wanting for this purpose.
View, for instance, the 19th cluster of my 6th class, and afterwards
cast your eye on this cloudy star, and the result will be no less
decisive than that of the naturalist we have alluded to. Our judgment, I
may venture to say, will be, that _the nebulosity about the star is not
of a starry nature_."[51]
The conviction thus arrived at of the existence in space of a widely
diffused "shining fluid" (a conviction long afterwards fully justified
by the spectroscope) led him into a field of endless speculation. What
was its nature? Should it "be compared to the coruscation of the
electric fluid in the aurora borealis? or to the more magnificent cone
of the zodiacal light?" Above all, what was its function in the cosmos?
And on this point he already gave a hint of the direction in which his
mind was moving by the remark that this self-luminous matter seemed
"more fit to produce a star by its condensation, than to depend on the
star for its existence."[52]
This was not a novel idea. Tycho Brahe had tried to explain the blaze of
the star of 1572 as due to a sudden concentration of nebulous material
in the Milky Way, even pointing out the space left dark and void by the
withdrawal of the luminous stuff; and Kepler, theorising on a similar
stellar apparition in 1604, followed nearly in the same track. But under
Herschel's treatment the nebular origin of stars first acquired the
consistency of a formal theory. He meditated upon it long and earnestly,
and in two elaborate treatises, published respectively in 1811 and 1814,
he at length set forth the arguments in its favour. These rested
entirely upon the "principle of continuity." Between the successive
classes of his assortment of developing objects there was, as he said,
"perhaps not so much difference as would be in an annual description of
the human figure, were it given from the birth of a child till he comes
to be a man in his prime."[53] From diffused nebulosity, barely visible
in the most powerful light-gathering instruments, but which he estimated
to cover nearly 152 square degrees of the heavens,[54] to planetary
nebulæ, supposed to be already centrally solid, instances were alleged
of every stage and phase of condensation. The validity of his reasoning,
however, was evidently impaired by his confessed inability to
distinguish between the dim rays of remote clusters and the milky light
of true gaseous nebulæ.
It may be said that such speculations are futile in themselves, and
necessarily barren of results. But they gratify an inherent tendency of
the human mind, and, if pursued in a becoming spirit, should be neither
reproved nor disdained. Herschel's theory still holds the field, the
testimony of recent discoveries with regard to it having proved strongly
confirmatory of its principle, although not of its details. Strangely
enough, it seems to have been propounded in complete independence of
Laplace's nebular hypothesis as to the origin of the solar system.
Indeed, it dated, as we have seen, in its first inception, from 1791,
while the French geometrician's view was not advanced until 1796.
We may now briefly sum up the chief results of Herschel's long years of
"watching the heavens." The apparent motions of the stars had been
disentangled; one portion being clearly shown to be due to a translation
towards a point in the constellation Hercules of the sun and his
attendant planets; while a large balance of displacement was left to be
accounted for by real movements, various in extent and direction, of the
stars themselves. By the action of a central force similar to, if not
identical with, gravity, suns of every degree of size and splendour, and
sometimes brilliantly contrasted in colour, were seen to be held
together in systems, consisting of two, three, four, even six members,
whose revolutions exhibited a wide range of variety both in period and
in orbital form. A new department of physical astronomy was thus
created,[55] and rigid calculation for the first time made possible
within the astral region. The vast problem of the arrangement and
relations of the millions of stars forming the Milky Way was shown to be
capable of experimental treatment, and of at least partial solution,
notwithstanding the variety and complexity seen to prevail, to an extent
previously undreamt of, in the arrangement of that majestic system. The
existence of a luminous fluid, diffused through enormous tracts of
space, and intimately associated with stellar bodies, was virtually
demonstrated, and its place and use in creation attempted to be divined
by a bold but plausible conjecture. Change on a stupendous scale was
inferred or observed to be everywhere in progress. Periodical stars
shone out and again decayed; progressive ebbings or flowings of light
were indicated as probable in many stars under no formal suspicion of
variability; forces were everywhere perceived to be at work, by which
the very structure of the heavens themselves must be slowly but
fundamentally modified. In all directions groups were seen to be formed
or forming; tides and streams of suns to be setting towards powerful
centres of attraction; new systems to be in process of formation, while
effete ones hastened to decay or regeneration when the course appointed
for them by Infinite Wisdom was run. And thus, to quote the words of the
observer who "had looked farther into space than ever human being did
before him,"[56] the state into which the incessant action of the
clustering power has brought the Milky Way at present, is a kind of
chronometer that may be used to measure the time of its past and future
existence; and although we do not know the rate of going of this
mysterious chronometer, it is nevertheless certain that, since the
breaking-up of the parts of the Milky Way affords a proof that it cannot
last for ever, it equally bears witness that its past duration cannot be
admitted to be infinite.[57]
FOOTNOTES:
[Footnote 3: _Phil. Trans._, vol. xxx., p. 737.]
[Footnote 4: Out of eighty stars compared, fifty-seven were found to
have changed their places by more than 10". Lesser discrepancies were at
that time regarded as falling within the limits of observational error.
_Tobiæ Mayeri Op. Inedita_, t. i., pp. 80, 81, and Herschel in _Phil.
Trans._, vol. lxxiii., pp. 275-278.]
[Footnote 5: _Posthumous Works_, p. 701.]
[Footnote 6: Arago in _Annuaire du Bureau des Longitudes_, 1842, p.
313.]
[Footnote 7: Bradley to Halley, _Phil. Trans._, vol. xxxv. (1728), p.
660. His observations were directly applicable to only two stars,
Gamma Draconis and Eta Ursæ Majoris, but some lesser ones
were included in the same result.]
[Footnote 8: Holden, _Sir William Herschel, his Life and Works_, p. 17.]
[Footnote 9: _Phil. Trans._, vol. ci., p. 269.]
[Footnote 10: Caroline Lucretia Herschel, born at Hanover, March 16,
1750, died in the same place, January 9, 1848. She came to England in
1772, and was her brother's devoted assistant, first in his musical
undertakings, and afterwards, down to the end of his life, in his
astronomical labours.]
[Footnote 11: Holden, _op. cit._, p. 39.]
[Footnote 12: _Memoir of Caroline Herschel_, p. 37.]
[Footnote 13: See Holden's _Sir William Herschel_, p. 54.]
[Footnote 14: _An Original Theory or New Hypothesis of the Universe_,
London, 1750. See also De Morgan's summary of his views in
_Philosophical Magazine_, April, 1848.]
[Footnote 15: _Allgemeine Naturgeschichte und Theorie des Himmels_,
1755.]
[Footnote 16: _Cosmologische Briefe_, Augsburg, 1761.]
[Footnote 17: _The System of the World_, p. 125, London, 1800 (a
translation of _Cosmologische Briefe_). Lambert regarded nebulæ as
composed of stars crowded together, but _not_ as external universes. In
the case of the Orion nebula, indeed, he throws out such a conjecture,
but afterwards suggests that it may form a centre for that one of the
subordinate systems composing the Milky Way to which our sun belongs.]
[Footnote 18: _Opera Inedita_, t. i., p. 79.]
[Footnote 19: _Phil. Trans._, vol. lxxiii. (1783), p. 273. Pierre
Prévost's similar investigation, communicated to the Berlin Academy of
Sciences four months later, July 3, 1783, was inserted in the _Memoirs_
of that body for 1781, and thus _seems_ to claim a priority not its due.
Georg Simon Klügel at Halle gave about the same time an analytical
demonstration of Herschel's result. Wolf, _Gesch. der Astronomie_, p.
733.]
[Footnote 20: _Phil. Trans._, vol. xcv., p. 233.]
[Footnote 21: _Ibid._, vol. xcvi., p. 205.]
[Footnote 22: "Ingens bolus devorandus est," Kepler admitted to Herwart
in May, 1603.]
[Footnote 23: Described in "Præfatio Editoris" to _De Revolutionibus_,
p. xix. (ed. 1854).]
[Footnote 24: _Opere_, t. i., p. 415.]
[Footnote 25: _Phil. Trans._, vol. xvii., p. 848.]
[Footnote 26: _Ibid._, vol. lxxii., p. 97.]
[Footnote 27: Doberck, _Observatory_, vol. ii., p. 110.]
[Footnote 28: _Phil. Trans._, vol. lvii., p. 249.]
[Footnote 29: _Ibid._, vol. lxxiv., p. 56.]
[Footnote 30: _Beobachtungen von Fixsterntrabanten_, 1778; and _De Novis
in Coelo Sidereo Phænomenis_, 1779.]
[Footnote 31: _Bibliographie_, p. 569.]
[Footnote 32: _Phil. Trans._, vol. lxxii., p. 162.]
[Footnote 33: _Ibid._, vol. lxxiii., p. 272.]
[Footnote 34: _Ibid._, vol. xciii., p. 340.]
[Footnote 35: _Phil. Trans._, vol. lxxv., p. 255.]
[Footnote 36: _Ibid._, vol. lxxix., pp. 214, 222.]
[Footnote 37: _Ibid._, vol. xcii., pp. 479, 495.]
[Footnote 38: _Phil. Trans._, vol. ci., p. 269.]
[Footnote 39: _Ibid._, vol. cvii., p. 311.]
[Footnote 40: Bullialdus, _De Nebulosâ Stellâ in Cingulo Andromedæ_
(1667); see also G. P. Bond, _Mém. Am. Ac._, vol. iii., p. 75, Holden's
Monograph on the Orion Nebula, _Washington Observations_, vol. xxv.,
1878 (pub. 1882), and Lady Huggins's drawing, _Atlas of Spectra_, p.
119.]
[Footnote 41: _Mathemata Astronomica_, p. 75.]
[Footnote 42: _Systema Saturnium_, p. 9.]
[Footnote 43: _Phil. Trans._, vol. xxix., p. 390.]
[Footnote 44: _Mém. Ac. des Sciences_, 1755.]
[Footnote 45: _Conn. des Temps_, 1784 (pub. 1781), p. 227. A previous
list of forty-five had appeared in _Mém. Ac. des Sciences_, 1771.]
[Footnote 46: _Phil. Trans._, vol. lxxiv., p. 442.]
[Footnote 47: _Ibid._, vol. lxxix., p. 213.]
[Footnote 48: _Ibid._, vol. lxxv., p. 254.]
[Footnote 49: _Ibid._, vol. lxxix., p. 225.]
[Footnote 50: _Phil. Trans._, vol. lxxix., p. 226.]
[Footnote 51: _Ibid._, vol. lxxxi., p. 72.]
[Footnote 52: _Ibid._, p. 85.]
[Footnote 53: _Phil. Trans._, vol. ci., p. 271.]
[Footnote 54: _Ibid._, p. 277.]
[Footnote 55: J. Herschel, _Phil. Trans._, vol. cxvi., part iii., p. 1.]
[Footnote 56: His own words to the poet Campbell cited by Holden, _Life
and Works_, p. 109.]
[Footnote 57: _Phil. Trans._, vol. civ., p. 283.]
CHAPTER II
_PROGRESS OF SIDEREAL ASTRONOMY_
We have now to consider labours of a totally different character from
those of Sir William Herschel. Exploration and discovery do not
constitute the whole business of astronomy; the less adventurous, though
not less arduous, task of gaining a more and more complete mastery over
the problems immemorially presented to her, may, on the contrary, be
said to form her primary duty. A knowledge of the movements of the
heavenly bodies has, from the earliest times, been demanded by the
urgent needs of mankind; and science finds its advantage, as in many
cases it has taken its origin, in condescension to practical claims.
Indeed, to bring such knowledge as near as possible to absolute
precision has been defined by no mean authority[58] as the true end of
astronomy.
Several causes concurred about the beginning of the last century to give
a fresh and powerful impulse to investigations having this end in view.
The rapid progress of theory almost compelled a corresponding advance in
observation; instrumental improvements rendered such an advance
possible; Herschel's discoveries quickened public interest in celestial
inquiries; royal, imperial, and grand-ducal patronage widened the scope
of individual effort. The heart of the new movement was in Germany.
Hitherto the observatory of Flamsteed and Bradley had been the
acknowledged centre of practical astronomy; Greenwich observations were
the standard of reference all over Europe; and the art of observing
prospered in direct proportion to the fidelity with which Greenwich
methods were imitated. Dr. Maskelyne, who held the post of Astronomer
Royal during forty-six years (from 1765 to 1811), was no unworthy
successor to the eminent men who had gone before him. His foundation of
the _Nautical Almanac_ (in 1767) alone constitutes a valid title to
fame; he introduced at the Observatory the important innovation of the
systematic publication of results; and the careful and prolonged series
of observations executed by him formed the basis of the improved
theories, and corrected tables of the celestial movements, which were
rapidly being brought to completion abroad. His catalogue of thirty-six
"fundamental" stars was besides excellent in its way, and most
serviceable. Yet he was devoid of Bradley's instinct for divining the
needs of the future. He was fitted rather to continue a tradition than
to found a school. The old ways were dear to him; and, indefatigable as
he was, a definite purpose was wanting to compel him, by its exigencies,
along the path of progress. Thus, for almost fifty years after Bradley's
death, the acquisition of a small achromatic[59] was the only notable
change made in the instrumental equipment of the Observatory. The
transit, the zenith sector, and the mural quadrant, with which Bradley
had done his incomparable work, retained their places long after they
had become deteriorated by time and obsolete by the progress of
invention; and it was not until the very close of his career that
Maskelyne, compelled by Pond's detection of serious errors, ordered a
Troughton's circle, which he did not live to employ.
Meanwhile, the heavy national disasters with which Germany was
overwhelmed in the early part of the nineteenth century seemed to
stimulate rather than impede the intellectual revival already for some
years in progress there. Astronomy was amongst the first of the sciences
to feel the new impulse. By the efforts of Bode, Olbers, Schröter, and
Von Zach, just and elevated ideas on the subject were propagated,
intelligence was diffused, and a firm ground prepared for common action
in mutual sympathy and disinterested zeal. They received powerful aid
through the foundation, in 1804, by a young artillery officer named Von
Reichenbach, of an Optical and Mechanical Institute at Munich. Here the
work of English instrumental artists was for the first time rivalled,
and that of English opticians--when Fraunhofer entered the new
establishment--far surpassed. The development given to the refracting
telescope by this extraordinary man was indispensable to the progress of
that fundamental part of astronomy which consists in the exact
determination of the places of the heavenly bodies. Reflectors are
brilliant engines of discovery, but they lend themselves with difficulty
to the prosaic work of measuring right ascensions and polar distances. A
signal improvement in the art of making and working flint-glass thus
most opportunely coincided with the rise of a German school of
scientific mechanicians, to furnish the instrumental means needed for
the reform which was at hand. Of the leader of that reform it is now
time to speak.
Friedrich Wilhelm Bessel was born at Minden, in Westphalia, July 22,
1784. A certain taste for figures, coupled with a still stronger
distaste for the Latin accidence, directed his inclination and his
father's choice towards a mercantile career. In his fifteenth year,
accordingly, he entered the house of Kuhlenkamp and Sons, in Bremen, as
an apprenticed clerk. He was now thrown completely upon his own
resources. From his father, a struggling Government official, heavily
weighted with a large family, he was well aware that he had nothing to
expect; his dormant faculties were roused by the necessity for
self-dependence, and he set himself to push manfully forward along the
path that lay before him. The post of supercargo on one of the trading
expeditions sent out from the Hanseatic towns to China and the East
Indies was the aim of his boyish ambition, for the attainment of which
he sought to qualify himself by the industrious acquisition of suitable
and useful knowledge. He learned English in two or three months; picked
up Spanish with the casual aid of a gunsmith's apprentice; studied the
geography of the distant lands which he hoped to visit; collected
information as to their climates, inhabitants, products, and the courses
of trade. He desired to add some acquaintance with the art (then much
neglected) of taking observations at sea; and thus, led on from
navigation to astronomy, and from astronomy to mathematics, he groped
his way into a new world.
It was characteristic of him that the practical problems of science
should have attracted him before his mind was as yet sufficiently
matured to feel the charm of its abstract beauties. His first attempt at
observation was made with a sextant, rudely constructed under his own
directions, and a common clock. Its object was the determination of the
longitude of Bremen, and its success, he tells us himself,[60] filled
him with a rapture of delight, which, by confirming his tastes, decided
his destiny. He now eagerly studied Bode's _Jahrbuch_ and Von Zach's
_Monatliche Correspondenz_, overcoming each difficulty as it arose with
the aid of Lalande's _Traité d'Astronomie_, and supplying, with amazing
rapidity, his early deficiency in mathematical training. In two years he
was able to attack a problem which would have tasked the patience, if
not the skill, of the most experienced astronomer. Amongst the Earl of
Egremont's papers Von Zach had discovered Harriot's observations on
Halley's comet at its appearance in 1607, and published them as a
supplement to Bode's Annual. With an elaborate care inspired by his
youthful ardour, though hardly merited by their loose nature, Bessel
deduced from them an orbit for that celebrated body, and presented the
work to Olbers, whose reputation in cometary researches gave a special
fitness to the proffered homage. The benevolent physician-astronomer of
Bremen welcomed with surprised delight such a performance emanating from
such a source. Fifteen years previously, the French Academy had crowned
a similar work; now its equal was produced by a youth of twenty, busily
engaged in commercial pursuits, self-taught, and obliged to snatch from
sleep the hours devoted to study. The paper was immediately sent to Von
Zach for publication, with a note from Olbers explaining the
circumstances of its author; and the name of Bessel became the common
property of learned Europe.
He had, however, as yet no intention of adopting astronomy as his
profession. For two years he continued to work in the counting-house by
day, and to pore over the _Mécanique Céleste_ and the Differential
Calculus by night. But the post of assistant in Schröter's observatory
at Lilienthal having become vacant by the removal of Harding to
Göttingen in 1805, Olbers procured for him the offer of it. It was not
without a struggle that he resolved to exchange the desk for the
telescope. His reputation with his employers was of the highest; he had
thoroughly mastered the details of the business, which his keen
practical intelligence followed with lively interest; his years of
apprenticeship were on the point of expiring, and an immediate, and not
unwelcome prospect of comparative affluence lay before him. The love of
science, however, prevailed; he chose poverty and the stars, and went to
Lilienthal with a salary of a hundred thalers yearly. Looking back over
his life's work, Olbers long afterwards declared that the greatest
service which he had rendered to astronomy was that of having discerned,
directed, and promoted the genius of Bessel.[61]
For four years he continued in Schröter's employment. At the end of that
time the Prussian Government chose him to superintend the erection of a
new observatory at Königsberg, which after many vexatious delays, caused
by the prostrate condition of the country, was finished towards the end
of 1813. Königsberg was the first really efficient German observatory.
It became, moreover, a centre of improvement, not for Germany alone, but
for the whole astronomical world. During two-and-thirty years it was the
scene of Bessel's labours, and Bessel's labours had for their aim the
reconstruction, on an amended and uniform plan, of the entire science of
observation.
A knowledge of the places of the stars is the foundation of
astronomy.[62] Their configuration lends to the skies their distinctive
features, and marks out the shifting tracks of more mobile objects with
relatively fixed, and generally unvarying points of light. A more
detailed and accurate acquaintance with the stellar multitude, regarded
from a purely uranographical point of view, has accordingly formed at
all times a primary object of celestial science, and was, during the
last century, cultivated with a zeal and success by which all previous
efforts were dwarfed into insignificance. In Lalande's _Histoire
Céleste_, published in 1801, the places of no less than 47,390 stars
were given, but in the rough, as it were, and consequently needing
laborious processes of calculation to render them available for exact
purposes. Piazzi set an example of improved methods of observation,
resulting in the publication, in 1803 and 1814, of two catalogues of
about 7,600 stars--the second being a revision and enlargement of the
first--which for their time were models of what such works should
be.[63] Stephen Groombridge at Blackheath was similarly and most
beneficially active. But something more was needed than the diligence of
individual observers. A systematic reform was called for; and it was
this which Bessel undertook and carried through.
Direct observation furnishes only what has been called the "raw
material" of the positions of the heavenly bodies.[64] A number of
highly complex corrections have to be applied before their _mean_ can be
disengaged from their _apparent_ places on the sphere. Of these, the
most considerable and familiar is atmospheric refraction, by which
objects seem to stand higher in the sky than they in reality do, the
effect being evanescent at the zenith, and attaining, by gradations
varying with conditions of pressure and temperature, a maximum at the
horizon. Moreover, the points to which measurements are referred are
themselves in motion, either continually in one direction, or
periodically to and fro. The _precession_ of the equinoxes is slowly
progressive, or rather retrogressive; the _nutation_ of the pole
oscillatory in a period of about eighteen years. Added to which, the
non-instantaneous transmission of light, combined with the movement of
the earth in its orbit, causes a small annual displacement known as
_aberration_.
Now it is easy to see that any uncertainty in the application of these
corrections saps the very foundations of exact astronomy. Extremely
minute quantities, it is true, are concerned; but the life and progress
of modern celestial science depends upon the sure recognition of
extremely minute quantities. In the early years of the nineteenth
century, however, no uniform system of "reduction" (so the complete
correction of observational results is termed) had been established.
Much was left to the individual caprice of observers, who selected for
the several "elements" of reduction such values as seemed best to
themselves. Hence arose much hurtful confusion, tending to hinder united
action and mar the usefulness of laborious researches. For this state of
things, Bessel, by the exercise of consummate diligence, sagacity, and
patience, provided an entirely satisfactory remedy.
His first step was an elaborate investigation of the precious series of
observations made by Bradley at Greenwich from 1750 until his death in
1762. The catalogue of 3,222 stars which he extracted from them gave the
earliest example of the systematic reduction on a uniform plan of such a
body of work. It is difficult, without entering into details out of
place in a volume like the present, to convey an idea of the arduous
nature of this task. It involved the formation of a theory of the errors
of each of Bradley's instruments, and a difficult and delicate inquiry
into the true value of each correction to be applied, before the entries
in the Greenwich journals could be developed into a finished and
authentic catalogue. Although completed in 1813, it was not until five
years later that the results appeared with the proud, but not
inappropriate title of _Fundamenta Astronomiæ_. The eminent value of the
work consisted in this, that by providing a mass of entirely reliable
information as to the state of the heavens at the epoch 1755, it threw
back the beginning of _exact_ astronomy almost half a century. By
comparison with Piazzi's catalogues the amount of precession was more
accurately determined, the proper motions of a considerable number of
stars became known with certainty, and definite prediction--the
certificate of initiation into the secrets of Nature--at last became
possible as regards the places of the stars. Bessel's final improvements
in the methods of reduction were published in 1830 in his _Tabulæ
Regiomontanæ_. They not only constituted an advance in accuracy, but
afforded a vast increase of facility in application, and were at once
and everywhere adopted. Thus astronomy became a truly universal science;
uncertainties and disparities were banished, and observations made at
all times and places rendered mutually comparable.[65]
More, however, yet remained to be done. In order to verify with greater
strictness the results drawn from the Bradley and Piazzi catalogues, a
third term of comparison was wanted, and this Bessel undertook to
supply. By a course of 75,011 observations, executed during the years
1821-33, with the utmost nicety of care, the number of accurately known
stars was brought up to above 50,000, and an ample store of trustworthy
facts laid up for the use of future astronomers. In this department
Argelander, whom he attracted from finance to astronomy, and trained in
his own methods, was his assistant and successor. The great "Bonn
Durchmusterung,"[66] in which 324,198 stars visible in the northern
hemisphere are enumerated, and the corresponding "Atlas" published in
1857-63, constituting a picture of our sidereal surroundings of
heretofore unapproached completeness, may be justly said to owe their
origin to Bessel's initiative, and to form a sequel to what he
commenced.
But his activity was not solely occupied with the promotion of a
comprehensive reform in astronomy; it embraced special problems as well.
The long-baffled search for a parallax of the fixed stars was resumed
with fresh zeal as each mechanical or optical improvement held out fresh
hopes of a successful issue. Illusory results abounded. Piazza in 1805
perceived, as he supposed, considerable annual displacements in Vega,
Aldebaran, Sirius, and Procyon; the truth being that his instruments
were worn out with constant use, and could no longer be depended
upon.[67] His countryman, Calandrelli, was similarly deluded. The
celebrated controversy between the Astronomer Royal and Dr. Brinkley,
Director of the Dublin College Observatory, turned on the same subject.
Brinkley, who was in possession of a first-rate meridian-circle,
believed himself to have discovered relatively large parallaxes for four
of the brightest stars; Pond, relying on the testimony of the Greenwich
instruments, asserted their nullity. The dispute, protracted for
fourteen years, from 1810 until 1824, was brought to no definite
conclusion; but the strong presumption on the negative side was
abundantly justified in the event.
There was good reason for incredulity in the matter of parallaxes.
Announcements of their detection had become so frequent as to be
discredited before they were disproved; and Struve, who investigated the
subject at Dorpat in 1818-21, had clearly shown that the quantities
concerned were too small to come within the reliable measuring powers of
any instrument then in use. Already, however, the means were being
prepared of giving to those powers a large increase.
On the 21st July, 1801, two old houses in an alley of Munich tumbled
down, burying in their ruins the occupants, of whom one alone was
extricated alive, though seriously injured. This was an orphan lad of
fourteen named Joseph Fraunhofer. The Elector Maximilian Joseph was
witness of the scene, became interested in the survivor, and consoled
his misfortune with a present of eighteen ducats. Seldom was money
better bestowed. Part of it went to buy books and a glass-polishing
machine, with the help of which young Fraunhofer studied mathematics and
optics, and secretly exercised himself in the shaping and finishing of
lenses; the remainder purchased his release from the tyranny of one
Weichselberger, a looking-glass maker by trade, to whom he had been
bound apprentice on the death of his parents. A period of struggle and
privation followed, during which, however, he rapidly extended his
acquirements; and was thus eminently fitted for the task awaiting him,
when, in 1806, he entered the optical department of the establishment
founded two years previously by Von Reichenbach and Utzschneider. He now
zealously devoted himself to the improvement of the achromatic
telescope; and, after a prolonged study of the theory of lenses, and
many toilsome experiments in the manufacture of flint-glass, he
succeeded in perfecting, December 12, 1817, an object-glass of exquisite
quality and finish, 9-1/2 inches in diameter, and of 14 feet focal
length.
This (as it was then considered) gigantic lens was secured by Struve for
the Russian Government, and the "great Dorpat refractor"--the first of
the large achromatics which have played such an important part in modern
astronomy--was, late in 1824, set up in the place which it still
occupies. By ingenious improvements in mounting and fitting, it was
adapted to the finest micrometrical work, and thus offered unprecedented
facilities both for the examination of double stars (in which Struve
chiefly employed it), and for such subtle measurements as might serve to
reveal or disprove the existence of a sensible stellar parallax.
Fraunhofer, moreover, constructed for the observatory at Königsberg the
first really available heliometer. The principle of this instrument
(termed with more propriety a "divided object-glass micrometer") is the
separation, by a strictly measurable amount, of two distinct images of
the same object. If a double star, for instance, be under examination,
the two half-lenses into which the object-glass is divided are shifted
until the upper star (say) in one image is brought into coincidence with
the lower star in the other, when their distance apart becomes known by
the amount of motion employed.[68]
This virtually new engine of research was delivered and mounted in 1829,
three years after the termination of the life of its deviser. The Dorpat
lens had brought to Fraunhofer a title of nobility and the sole
management of the Munich Optical Institute (completely separated since
1814 from the mechanical department). What he had achieved, however, was
but a small part of what he meant to achieve. He saw before him the
possibility of nearly quadrupling the light-gathering capacity of the
great achromatic acquired by Struve; he meditated improvements in
reflectors as important as those he had already effected in refractors;
and was besides eagerly occupied with investigations into the nature of
light, the momentous character of which we shall by-and-by have an
opportunity of estimating. But his health was impaired, it is said, from
the weakening effects of his early accident, combined with excessive and
unwholesome toil, and, still hoping for its restoration from a projected
journey to Italy, he died of consumption, June 7, 1826, aged thirty-nine
years. His tomb in Munich bears the concise eulogy, _Approximavit
sidera_.
Bessel had no sooner made himself acquainted with the exquisite defining
powers of the Königsberg heliometer, than he resolved to employ them in
an attack upon the now secular problem of star-distances. But it was not
until 1837 that he found leisure to pursue the inquiry. In choosing his
test-star he adopted a new principle. It had hitherto been assumed that
our nearest neighbours in space must be found among the brightest
ornaments of our skies. The knowledge of stellar proper motions afforded
by the critical comparison of recent with earlier star-places, suggested
a different criterion of distance. It is impossible to escape from the
conclusion that the apparently swiftest-moving stars are, _on the
whole_, also the nearest to us, however numerous the individual
exceptions to the rule. Now, as early as 1792,[69] Piazzi had noted as
an indication of relative vicinity to the earth, the unusually large
proper motion (5·2" annually) of a double star of the fifth magnitude in
the constellation of the Swan. Still more emphatically in 1812[70]
Bessel drew the attention of astronomers to the fact, and 61 Cygni
became known as the "flying star." The _seeming_ rate of its flight,
indeed, is of so leisurely a kind, that in a thousand years it will have
shifted its place by less than 3-1/2 lunar diameters, and that a quarter
of a million would be required to carry it round the entire circuit of
the visible heavens. Nevertheless, it has few rivals in rapidity of
movement, the apparent displacement of the vast majority of stars being,
by comparison, almost insensible.
This interesting, though inconspicuous object, then, was chosen by
Bessel to be put to the question with his heliometer, while Struve made
a similar and somewhat earlier trial with the bright gem of the Lyre,
whose Arabic title of the "Falling Eagle" survives as a time-worn
remnant in "Vega." Both astronomers agreed to use the "differential"
method, for which their instruments and the vicinity to their selected
stars of minute, physically detached companions offered special
facilities. In the last month of 1838 Bessel made known the result of
one year's observations, showing for 61 Cygni a parallax of about a
third of a second (0·3136").[71] He then had his heliometer taken down
and repaired, after which he resumed the inquiry, and finally terminated
a series of 402 measures in March 1840.[72] The resulting parallax of
0·3483" (corresponding to a distance about 600,000 times that of the
earth from the sun), seemed to be ascertained beyond the possibility of
cavil, and is memorable as the first _published_ instance of the
fathom-line, so industriously thrown into celestial space, having really
and indubitably _touched bottom_. It was confirmed in 1842-43 with
curious exactness by C. A. F. Peters at Pulkowa; but later researches
showed that it required increase to nearly half a second.[73]
Struve's measurements inspired less confidence. They extended over three
years (1835-38), but were comparatively few, and were frequently
interrupted. The parallax, accordingly, of about a quarter of a second
(0·2613") which he derived from them for Alpha Lyræ, and announced
in 1840,[74] has proved considerably too large.[75]
Meanwhile a result of the same kind, but of a more striking character
than either Bessel's or Struve's, had been obtained, one might almost
say casually, by a different method and in a distant region. Thomas
Henderson, originally an attorney's clerk in his native town of Dundee,
had become known for his astronomical attainments, and was appointed in
1831 to direct the recently completed observatory at the Cape of Good
Hope. He began observing in April, 1832, and, the serious shortcomings
of his instrument notwithstanding, executed during the thirteen months
of his tenure of office a surprising amount of first-rate work. With a
view to correcting the declination of the lustrous double star Alpha
Centauri (which ranks after Sirius and Canopus as the third brightest
orb in the heavens), he effected a number of successive determinations
of its position, and on being informed of its very considerable proper
motion (3·6" annually), he resolved to examine the observations already
made for possible traces of parallactic displacement. This was done on
his return to Scotland, where he filled the office of Astronomer Royal
from 1834 until his premature death in 1844. The result justified his
expectations. From the declination measurements made at the Cape and
duly reduced, a parallax of about one second of arc clearly emerged
(diminished by Gill's and Elkin's observations, 1882-1883, to O·75");
but, by perhaps an excess of caution, was withheld from publication
until fuller certainty was afforded by the concurrent testimony of
Lieutenant Meadows's determinations of the same star's right
ascension.[76] When at last, January 9, 1839, Henderson communicated his
discovery to the Astronomical Society, he could no longer claim the
priority which was his due. Bessel had anticipated him with the parallax
of 61 Cygni by just two months.
Thus from three different quarters, three successful and almost
simultaneous assaults were delivered upon a long-beleaguered citadel of
celestial secrets. The same work has since been steadily pursued, with
the general result of showing that, as regards their overwhelming
majority, the stars are far too remote to show even the slightest trace
of optical shifting from the revolution of the earth in its orbit. In
nearly a hundred cases, however, small parallaxes have been determined,
some certainly (that is, within moderate limits of error), others more
or less precariously. The list is an instructive one, in its omissions
no less than in its contents. It includes stars of many degrees of
brightness, from Sirius down to a nameless telescopic star in the Great
Bear;[77] yet the vicinity to the earth of this minute object is so much
greater than that of the brilliant Vega, that the latter transported to
its place would increase in lustre thirty-eight times. Moreover, many of
the brightest stars are found to have no sensible parallax, while the
majority of those ascertained to be nearest to the earth are of fifth,
sixth, even ninth magnitudes. The obvious conclusions follow that the
range of variety in the sidereal system is enormously greater than had
been supposed, and that estimates of distance based upon apparent
magnitude must be wholly futile. Thus, the splendid Canopus, Betelgeux,
and Rigel can be inferred, from their indefinite remoteness, to exceed
our sun thousands of times in size and lustre; while many inconspicuous
objects, which prove to be in our relative vicinity, must be notably his
inferiors. The limits of real stellar magnitude are then set very widely
apart. At the same time, the so-called "optical" and "geometrical"
methods of relatively estimating star-distances are both seen to have a
foundation of fact, although so disguised by complicated relations as to
be of very doubtful individual application. On the whole, the chances
are in favour of the superior vicinity of a bright star over a faint
one; and, on the whole, the stars in swiftest _apparent_ motion are
amongst those whose _actual_ remoteness is least. Indeed, there is no
escape from either conclusion, unless on the supposition of special
arrangements in themselves highly improbable, and, we may confidently
say, non-existent.
The distances even of the few stars found to have measurable parallaxes
are on a scale entirely beyond the powers of the human mind to conceive.
In the attempt both to realize them distinctly, and to express them
conveniently, a new unit of length, itself of bewildering magnitude, has
originated. This is what we may call the _light-journey_ of one year.
The subtle vibrations of the ether, propagated on all sides from the
surface of luminous bodies, travel at the rate of 186,300 miles a
second, or (in round numbers) six billions of miles a year. Four and a
third such measures are needed to span the abyss that separates us from
the nearest fixed star. In other words, light takes four years and four
months to reach the earth from Alpha Centauri; yet Alpha Centauri lies
some ten billions of miles nearer to us (so far as is yet known) than
any other member of the sidereal system!
The determination of parallax leads, in the case of stars revolving in
known orbits, to the determination of mass; for the distance from the
earth of the two bodies forming a binary system being ascertained, the
seconds of arc apparently separating them from each other can be
translated into millions of miles; and we only need to add a knowledge
of their period to enable us, by an easy sum in proportion, to find
their combined mass in terms of that of the sun. Thus, since--according
to Dr. Doberck's elements--the components of Alpha Centauri revolve
round their common centre of gravity at a mean distance nearly 25 times
the radius of the earth's orbit, in a period of 88 years, the attractive
force of the two together must be just twice the solar. We may gather
some idea of their relations by placing in imagination a second luminary
like our sun in circulation between the orbits of Neptune and Uranus.
But systems of still more majestic proportions are reduced by extreme
remoteness to apparent insignificance. A double star of the fourth
magnitude in Cassiopeia (Eta), to which a small parallax is ascribed on
the authority of O. Struve, appears to be above eight times as massive
as the central orb of our world; while a much less conspicuous pair--85
Pegasi--exerts, if the available data can be depended upon, no less than
thirteen times the solar gravitating power.
Further, the actual rate of proper motions, so far as regards that part
of them which is projected upon the sphere, can be ascertained for stars
at known distance. The annual journey, for instance, of 61 Cygni _across
the line of sight_ amounts to 1,000, and that of Alpha Centauri to
446 millions of miles. A small star, numbered 1,830 in Groombridge's
Circumpolar Catalogue, "devours the way" at the rate of at least 150
miles a second--a speed, in Newcomb's opinion, beyond the gravitating
power of the entire sidereal system to control; and Mu Cassiopeiæ
possesses above two-thirds of that surprising velocity; while for both
objects, radial movements of just sixty miles a second were disclosed
by Professor Campbell's spectroscopic measurements.
Herschel's conclusion as to the advance of the sun among the stars was
not admitted as valid by the most eminent of his successors. Bessel
maintained that there was absolutely no preponderating evidence in
favour of its supposed direction towards a point in the constellation
Hercules.[78] Biot, Burckhardt, even Herschel's own son, shared his
incredulity. But the appearance of Argelander's prize-essay in 1837[79]
changed the aspect of the question. Herschel's first memorable solution
in 1783 was based upon the motions of thirteen stars, imperfectly known;
his second, in 1805, upon those of no more than six. Argelander now
obtained an entirely concordant result from the large number of 390,
determined with the scrupulous accuracy characteristic of Bessel's work
and his own. The reality of the fact thus persistently disclosed could
no longer be doubted; it was confirmed five years later by the younger
Struve, and still more strikingly in 1847[80] by Galloway's
investigations, founded exclusively on the apparent displacements of
southern stars. In 1859 and 1863, Sir George Airy and Mr. Dunkin
(1821-1898),[81] employing all the resources of modern science, and
commanding the wealth of material furnished by 1,167 proper motions
carefully determined by Mr. Main, reached conclusions closely similar to
that indicated nearly eighty years previously by the first great
sidereal astronomer; which Mr. Plummer's reinvestigation of the subject
in 1883[82] served but slightly to modify. Yet astronomers were not
satisfied. Dr. Auwers of Berlin completed in 1866 a splendid piece of
work, for which he received in 1888 the Gold Medal of the Royal
Astronomical Society. It consisted in reducing afresh, with the aid of
the most refined modern data, Bradley's original stars, and comparing
their places thus obtained for the year 1755 with those assigned to them
from observations made at Greenwich after the lapse of ninety years. In
the interval, as was to be anticipated, most of them were found to have
travelled over some small span of the heavens, and there resulted a
stock of nearly three thousand highly authentic proper motions. These
ample materials were turned to account by M. Ludwig Struve[83] for a
discussion of the sun's motion, of which the upshot was to shift its
point of aim to the bordering region of the constellations Hercules and
Lyra. And the more easterly position of the solar apex was fully
confirmed by the experiments, with variously assorted lists of stars, of
Lewis Boss of Albany,[84] and Oscar Stumpe of Bonn.[85] Fresh
precautions of refinement were introduced into the treatment of the
subject by Ristenpart of Karlsruhe,[86] by Kapteyn of Groningen,[87] by
Newcomb[88] and Porter[89] in America, who ably availed themselves of
the copious materials accumulated before the close of the century. Their
results, although not more closely accordant than those of their
predecessors, combined to show that the journey of our system is
directed towards a point within a circle about ten degrees in radius,
having the brilliant Vega for its centre. To determine its rate was a
still more arduous problem. It involved the assumption, very much at
discretion, of an average parallax for the stars investigated; and Otto
Struve's estimate of 154 million miles as the span yearly traversed was
hence wholly unreliable. Fortunately, however, as will be seen further
on, a method of determining the sun's velocity independently of any
knowledge of star-distances, has now become available.
As might have been expected, speculation has not been idle regarding the
purpose and goal of the strange voyage of discovery through space upon
which our system is embarked; but altogether fruitlessly. The variety of
the conjectures hazarded in the matter is in itself a measure of their
futility. Long ago, before the construction of the heavens had as yet
been made the subject of methodical inquiry, Kant was disposed to regard
Sirius as the "central sun" of the Milky Way; while Lambert surmised
that the vast Orion nebula might serve as the regulating power of a
subordinate group including our sun. Herschel threw out the hint that
the great cluster in Hercules might prove to be the supreme seat of
attractive force;[90] Argelander placed his central body in the
constellation Perseus;[91] Fomalhaut, the brilliant of the Southern
Fish, was set in the post of honour by Boguslawski of Breslau. Mädler
(who succeeded Struve at Dorpat in 1839) concluded from a more formal
inquiry that the ruling power in the sidereal system resided, not in any
single prepondering mass, but in the centre of gravity of the
self-controlled revolving multitude.[92] In the former case (as we know
from the example of the planetary scheme), the stellar motions would be
most rapid near the centre; in the latter, they would become accelerated
with remoteness from it.[93] Mädler showed that no part of the heavens
could be indicated as a region of exceptionally swift movements, such as
would result from the presence of a gigantic (though possibly obscure)
ruling body; but that a community of extremely sluggish movements
undoubtedly existed in and near the group of the Pleiades, where,
accordingly, he placed the centre of gravity of the Milky Way.[94] The
bright star Alcyone thus became the "central sun," but in a purely
passive sense, its headship being determined by its situation at the
point of neutralisation of opposing tendencies, and of consequent rest.
By an avowedly conjectural method, the solar period of revolution round
this point was fixed at 18,200,000 years.
The scheme of sidereal government framed by the Dorpat astronomer was,
it may be observed, of the most approved constitutional type;
deprivation, rather than increase of influence accompanying the office
of chief dignitary. But while we are still ignorant, and shall perhaps
ever remain so, of the fundamental plan upon which the Galaxy is
organised, recent investigations tend more and more to exhibit it, not
as monarchical (so to speak), but as federative. The community of proper
motions detected by Mädler in the vicinity of the Pleiades may
accordingly possess a significance altogether different from what he
imagined.
Bessel's so-called "foundation of an Astronomy of the Invisible" now
claims attention.[95] His prediction regarding the planet Neptune does
not belong to the present division of our subject; a strictly analogous
discovery in the sidereal system was, however, also very clearly
foreshadowed by him. His earliest suspicions of non-uniformity in the
proper motion of Sirius dated from 1834; they extended to Procyon in
1840; and after a series of refined measurements with the new Repsold
circle, he announced in 1844 his conclusion that these irregularities
were due to the presence of obscure bodies round which the two bright
Dog-stars revolved as they pursued their way across the sphere.[96] He
even assigned to each an approximate period of half a century. "I adhere
to the conviction," he wrote later to Humboldt, "that Procyon and Sirius
form real binary systems, consisting of a visible and an invisible star.
There is no reason to suppose luminosity an essential quality of
cosmical bodies. The visibility of countless stars is no argument
against the invisibility of countless others."[97]
An inference so contradictory to received ideas obtained little credit,
until Peters found, in 1851,[98] that the apparent anomalies in the
movements of Sirius could be completely explained by an orbital
revolution in a period of fifty years. Bessel's prevision was destined
to be still more triumphantly vindicated. On the 31st of January, 1862,
while in the act of trying a new 18-inch refractor, Mr. Alvan G. Clark
(one of the celebrated firm of American opticians) actually discovered
the hypothetical Sirian companion in the precise position required by
theory. It has now been watched through nearly an entire revolution
(period 49·4 years), and proves to be very slightly luminous in
proportion to its mass. Its attractive power, in fact, is nearly half
that of its primary, while it emits only 1/10000th of its light. Sirius
itself, on the other hand, possesses a far higher radiative intensity
than our sun. It gravitates--admitting Sir David Gill's parallax of
0·38" to be exact--like two suns, but shines like twenty. Possibly it is
much distended by heat, and undoubtedly its atmosphere intercepts a very
much smaller proportion of its light than in stars of the solar class.
As regards Procyon, visual verification was awaited until November 13,
1896, when Professor Schaeberle, with the great Lick refractor, detected
the long-sought object in the guise of a thirteenth-magnitude star. Dr.
See's calculations[99] showed it to possess one-fifth the mass of its
primary, or rather more than half that of our sun.[100] Yet it gives
barely 1/20000th of the sun's light, so that it is still nearer to total
obscurity than the dusky satellite of Sirius. The period of forty years
assigned to the system by Auwers in 1862[101] appears to be singularly
exact.
But Bessel was not destined to witness the recognition of "the
invisible" as a legitimate and profitable field for astronomical
research. He died March 17, 1846, just six months before the discovery
of Neptune, of an obscure disease, eventually found to be occasioned by
an extensive fungus-growth in the stomach. The place which he left
vacant was not one easy to fill. His life's work might be truly
described as "epoch-making." Rarely indeed shall we find one who
reconciled with the same success the claims of theoretical and practical
astronomy, or surveyed the science which he had made his own with a
glance equally comprehensive, practical, and profound.
The career of Friedrich Georg Wilhelm Struve illustrates the maxim that
science _differentiates_ as it develops. He was, while much besides, a
specialist in double stars. His earliest recorded use of the telescope
was to verify Herschel's conclusion as to the revolving movement of
Castor, and he never varied from the predilection which this first
observation at once indicated and determined. He was born at Altona, of
a respectable yeoman family, April 15, 1793, and in 1811 took a degree
in philology at the new Russian University of Dorpat. He then turned to
science, was appointed in 1813 to a professorship of astronomy and
mathematics, and began regular work in the Dorpat Observatory just
erected by Parrot for Alexander I. It was not, however, until 1819 that
the acquisition of a 5-foot refractor by Troughton enabled him to take
the position-angles of double stars with regularity and tolerable
precision. The resulting catalogue of 795 stellar systems gave the
signal for a general resumption of the Herschelian labours in this
branch. His success, so far, and the extraordinary facilities for
observation afforded by the Fraunhofer achromatic encouraged him to
undertake, February 11, 1825, a review of the entire heavens down to 15°
south of the celestial equator, which occupied more than two years, and
yielded, from an examination of above 120,000 stars, a harvest of about
2,200 previously unnoticed composite objects. The ensuing ten years were
devoted to delicate and patient measurements, the results of which were
embodied in _Mensuræ Micrometricæ_, published at St. Petersburg in 1837.
This monumental work gives the places, angles of position, distances,
colours, and relative brightness of 3,112 double and multiple stars, all
determined with the utmost skill and care. The record is one which gains
in value with the process of time, and will for ages serve as a standard
of reference by which to detect change or confirm discovery.
It appears from Struve's researches that about one in forty of all stars
down to the ninth magnitude is composite, but that the proportion is
doubled in the brighter orders.[102] This he attributed to the
difficulty of detecting the faint companions of very remote orbs. It was
also noticed, both by him and Bessel, that double stars are in general
remarkable for large proper motions. Struve's catalogue included no star
of which the components were more than 32" apart, because beyond that
distance the chances of merely optical juxtaposition become
considerable; but the immense preponderance of extremely close over (as
it were) loosely yoked bodies is such as to demonstrate their physical
connection, even if no other proof were forthcoming. Many stars
previously believed to be single divided under the scrutiny of the
Dorpat refractor; while in some cases, one member of a supposed binary
system revealed itself as double, thus placing the surprised observer in
the unexpected presence of a triple group of suns. Five instances were
noted of two pairs lying so close together as to induce a conviction of
their mutual dependence;[103] besides which, 124 examples occurred of
triple, quadruple, and multiple combinations, the reality of which was
open to no reasonable doubt.[104]
It was first pointed out by Bessel that the fact of stars exhibiting a
common proper motion might serve as an unfailing test of their real
association into systems. This was, accordingly, one of the chief
criteria employed by Struve to distinguish true binaries from merely
optical couples. On this ground alone, 61 Cygni was admitted to be a
genuine double star; and it was shown that, although its components
appeared to follow almost strictly rectilinear paths, yet the
probability of their forming a connected pair is actually greater than
that of the sun rising to-morrow morning.[105] Moreover, this tie of an
identical movement was discovered to unite bodies[106] far beyond the
range of distance ordinarily separating the members of binary systems,
and to prevail so extensively as to lead to the conclusion that single
do not outnumber conjoined stars more than twice or thrice.[107]
In 1835 Struve was summoned by the Emperor Nicholas to superintend the
erection of a new observatory at Pulkowa, near St. Petersburg, destined
for the special cultivation of sidereal astronomy. Boundless resources
were placed at his disposal, and the institution created by him was
acknowledged to surpass all others of its kind in splendour, efficiency,
and completeness. Its chief instrumental glory was a refractor of
fifteen inches aperture by Merz and Mahler (Fraunhofer's successors),
which left the famous Dorpat telescope far behind, and remained long
without a rival. On the completion of this model establishment, August
19, 1839, Struve was installed as its director, and continued to fulfil
the important duties of the post with his accustomed vigour until 1858,
when illness compelled his virtual resignation in favour of his son Otto
Struve, born at Dorpat in 1819. He died November 23, 1864.
An inquiry into the laws of stellar distribution, undertaken during the
early years of his residence at Pulkowa, led Struve to confirm in the
main the inferences arrived at by Herschel as to the construction of the
heavens. According to his view, the appearance known as the Milky Way is
produced by a collection of irregularly condensed star-clusters, within
which the sun is somewhat eccentrically placed. The nebulous ring which
thus integrates the light of countless worlds was supposed by him to be
made up of stars scattered over a bent or "broken plane," or to lie in
two planes slightly inclined to each other, our system occupying a
position near their intersection.[108] He further attempted to show that
the limits of this vast assemblage must remain for ever shrouded from
human discernment, owing to the gradual extinction of light in its
passage through space,[109] and sought to confer upon this celebrated
hypothesis a definiteness and certainty far beyond the aspirations of
its earlier advocates, Chéseaux and Olbers; but arbitrary assumptions
vitiated his reasonings on this, as well as on some other points.[110]
In his special line as a celestial explorer of the most comprehensive
type, Sir William Herschel had but one legitimate successor, and that
successor was his son. John Frederick William Herschel was born at
Slough, March 17, 1792, graduated with the highest honours from St.
John's College, Cambridge, in 1813, and entered upon legal studies with
a view to being called to the Bar. But his share in an early compact
with Peacock and Babbage, "to do their best to leave the world wiser
than they found it," was not thus to be fulfilled. The acquaintance of
Dr. Wollaston decided his scientific vocation. Already, in 1816, we find
him reviewing some of his father's double stars; and he completed in
1820 the 18-inch speculum which was to be the chief instrument of his
investigations. Soon afterwards, he undertook, in conjunction with Mr.
(later Sir James) South, a series of observations, issuing in the
presentation to the Royal Society of a paper[111] containing
micrometrical measurements of 380 binary stars, by which the elder
Herschel's inferences of orbital motion were, in many cases, strikingly
confirmed. A star in the Northern Crown, for instance (Eta Coronæ),
had completed more than one entire circuit since its first discovery;
another, Tau Ophiuchi, had _closed up_ into apparent singleness; while
the motion of a third, Xi Ursæ Majoris, in an obviously eccentric orbit,
was so rapid as to admit of being traced and measured from month to month.
It was from the first confidently believed that the force retaining
double stars in curvilinear paths was identical with that governing the
planetary revolutions. But that identity was not ascertained until
Savary of Paris showed, in 1827,[112] that the movements of the
above-named binary in the Great Bear could be represented with all
attainable accuracy by an ellipse calculated on orthodox gravitational
principles with a period of 58-1/4 years. Encke followed at Berlin with
a still more elegant method; and Sir John Herschel, pointing out the
uselessness of analytical refinements where the data were necessarily so
imperfect, described in 1832 a graphical process by which "the aid of
the eye and hand" was brought in "to guide the judgment in a case where
judgment only, and not calculation, could be of any avail."[113]
Improved methods of the same kind were published by Dr. See in
1893,[114] and by Mr. Burnham in 1894;[115] and our acquaintance with
stellar orbits is steadily gaining precision, certainty, and extent.
In 1825 Herschel undertook, and executed with great assiduity during the
ensuing eight years, a general survey of the northern heavens, directed
chiefly towards the verification of his father's nebular discoveries.
The outcome was a catalogue of 2,306 nebulæ and clusters, of which 525
were observed for the first time, besides 3,347 double stars discovered
almost incidentally.[116] "Strongly invited," as he tells us himself,
"by the peculiar interest of the subject, and the wonderful nature of
the objects which presented themselves," he resolved to attempt the
completion of the survey in the southern hemisphere. With this noble
object in view, he embarked his family and instruments on board the
_Mount Stewart Elphinstone_, and, after a prosperous voyage, landed at
Cape Town on the 16th of January, 1834. Choosing as the scene of his
observations a rural spot under the shelter of Table Mountain, he began
regular "sweeping" on the 5th of March. The site of his great reflector
is now marked with an obelisk, and the name of Feldhausen has become
memorable in the history of science; for the four years' work done there
may truly be said to open the chapter of our knowledge as regards the
southern skies.
The full results of Herschel's journey to the Cape were not made public
until 1847, when a splendid volume[117] embodying them was brought out
at the expense of the Duke of Northumberland. They form a sequel to his
father's labours such as the investigations of one man have rarely
received from those of another. What the elder observer did for the
northern heavens, the younger did for the southern, and with generally
concordant results. Reviving the paternal method of "star-gauging," he
showed, from a count of 2,299 fields, that the Milky Way surrounds the
solar system as a complete annulus of minute stars; not, however, quite
symmetrically, since the sun was thought to lie somewhat nearer to those
portions visible in the southern hemisphere, which display a brighter
lustre and a more complicated structure than the northern branches. The
singular cosmical agglomerations known as the "Magellanic Clouds" were
now, for the first time, submitted to a detailed, though admittedly
incomplete, examination, the almost inconceivable richness and variety
of their contents being such that a lifetime might with great profit be
devoted to their study. In the Greater Nubecula, within a compass of
forty-two square degrees, Herschel reckoned 278 distinct nebulæ and
clusters, besides fifty or sixty outliers, and a large number of stars
intermixed with diffused nebulosity--in all, 919 catalogued objects,
and, for the Lesser Cloud, 244. Yet this was only the most conspicuous
part of what his twenty-foot revealed. Such an extraordinary
concentration of bodies so various led him to the inevitable conclusion
that "the Nubeculæ are to be regarded as systems _sui generis_, and
which have no analogues in our hemisphere."[118] He noted also the
blankness of surrounding space, especially in the case of Nubecula
Minor, "the access to which on all sides," he remarked, "is through a
desert;" as if the cosmical material in the neighbourhood had been swept
up and garnered in these mighty groups.[119]
Of southern double stars, he discovered and gave careful measurements of
2,102, and described 1,708 nebulæ, of which at least 300 were new. The
list was illustrated with a number of drawings, some of them extremely
beautiful and elaborate.
Sir John Herschel's views as to the nature of nebulæ were considerably
modified by Lord Rosse's success in "resolving" with his great
reflectors a crowd of these objects into stars. His former somewhat
hesitating belief in the existence of phosphorescent matter,
"disseminated through extensive regions of space in the manner of a
cloud or fog,"[120] was changed into a conviction that no valid
distinction could be established between the faintest wisp of cosmical
vapour just discernible in a powerful telescope, and the most brilliant
and obvious cluster. He admitted, however, an immense range of possible
variety in the size and mode of aggregation of the stellar constituents
of various nebulæ. Some might appear nebulous from the closeness of
their parts; some from their smallness. Others, he suggested, might be
formed of "discrete luminous bodies floating in a non-luminous
medium;"[121] while the annular kind probably consisted of "hollow
shells of stars."[122] That a physical, and not merely an optical,
connection unites nebulæ with the _embroidery_ (so to speak) of small
stars with which they are in many instances profusely decorated, was
evident to him, as it must be to all who look as closely and see as
clearly as he did. His description of No. 2,093 in his northern
catalogue as "a network or tracery of nebula following the lines of a
similar network of stars,"[123] would alone suffice to dispel the idea
of accidental scattering; and many other examples of a like import might
be quoted. The remarkably frequent occurrence of one or more minute
stars in the close vicinity of "planetary" nebulæ led him to infer their
dependent condition; and he advised the maintenance of a strict watch
for evidences of circulatory movements, not only over these supposed
stellar satellites, but also over the numerous "double nebulæ," in
which, as he pointed out, "all the varieties of double stars as to
distance, position, and relative brightness, have their counterparts."
He, moreover, investigated the subject of nebular distribution by the
simple and effectual method of graphic delineation or "charting," and
succeeded in showing that while a much greater uniformity of scattering
prevails in the southern than in the northern heavens, a condensation is
nevertheless perceptible about the constellations Pisces and Cetus,
roughly corresponding to the "nebular region" in Virgo by its vicinity
(within 20° or 30°) to the opposite pole of the Milky Way. He concluded
"that the nebulous system is distinct from the sidereal, though
involving, and perhaps to a certain extent intermixed with, the
latter."[124]
Towards the close of his residence at Feldhausen, Herschel was fortunate
enough to witness one of those singular changes in the aspect of the
firmament which occasionally challenge the attention even of the
incurious, and excite the deepest wonder of the philosophical observer.
Immersed apparently in the Argo nebula is a star denominated Eta
Carinæ. When Halley visited St. Helena in 1677, it seemed of the fourth
magnitude; but Lacaille in the middle of the following century, and
others after him, classed it as of the second. In 1827 the traveller
Burchell, being then at St. Paul, near Rio Janeiro, remarked that it had
unexpectedly assumed the first rank--a circumstance the more surprising
to him because he had frequently, when in Africa during the years 1811
to 1815, noted it as of only fourth magnitude. This observation,
however, did not become generally known until later. Herschel, on his
arrival at Feldhausen, registered the star as a bright second, and had
no suspicion of its unusual character until December 16, 1837, when he
suddenly perceived its light to be almost tripled. It then far outshone
Rigel in Orion, and on the 2nd of January following it very nearly
matched Alpha Centauri. From that date it declined; but a second
and even brighter maximum occurred in April, 1843, when Maclear, then
director of the Cape Observatory, saw it blaze out with a splendour
approaching that of Sirius. Its waxings and wanings were marked by
curious "trepidations" of brightness extremely perplexing to theory. In
1863 it had sunk below the fifth magnitude, and in 1869 was barely
visible to the naked eye; yet it was not until eighteen years later that
it touched a minimum of 7·6 magnitude. Soon afterwards a recovery of
brightness set in, but was not carried very far; and the star now shines
steadily as of the seventh magnitude, its reddish light contrasting
effectively with the silvery rays of the surrounding nebula. An attempt
to include its fluctuations within a cycle of seventy years[125] has
signally failed; the extent and character of the vicissitudes to which
it is subject stamping it rather as a species of connecting link between
periodic and temporary stars.[126]
Among the numerous topics which engaged Herschel's attention at the Cape
was that of relative stellar brightness. Having contrived an
"astrometer" in which an "artificial star," formed by the total
reflection of moonlight from the base of a prism, served as a standard
of comparison, he was able to estimate the lustre of the _natural_ stars
examined by the distances at which the artificial object appeared equal
respectively to each. He thus constructed a table of 191 of the
principal stars,[127] both in the northern and southern hemispheres,
setting forth the numerical values of their apparent brightness
relatively to that of Alpha Centauri, which he selected as a unit
of measurement. Further, the light of the full moon being found by him
to exceed that of his standard star 27,408 times, and Dr. Wollaston
having shown that the light of the full moon is to that of the sun as
1:801,072[128] (Zöllner made the ratio 1:618,000), it became possible to
compare stellar with solar radiance. Hence was derived, in the case of
the few stars at ascertained distances, a knowledge of real lustre.
Alpha Centauri, for example, emits less than twice, Capella one hundred
times as much light as our sun; while Arcturus, at its enormous
distance, must display the splendour of 1,300 such luminaries.
Herschel returned to England in the spring of 1838, bringing with him a
wealth of observation and discovery such as had perhaps never before
been amassed in so short a time. Deserved honours awaited him. He was
created a baronet on the occasion of the Queen's coronation (he had been
knighted in 1831); universities and learned societies vied with each
other in showering distinctions upon him; and the success of an
enterprise in which scientific zeal was tinctured with an attractive
flavour of adventurous romance, was justly regarded as a matter of
national pride. His career as an observing astronomer was now virtually
closed, and he devoted his leisure to the collection and arrangement of
the abundant trophies of his father's and his own activity. The
resulting great catalogue of 5,079 nebulæ (including all then certainly
known), published in the _Philosophical Transactions_ for 1864, is, and
will probably long remain, the fundamental source of information on the
subject;[129] but he unfortunately did not live to finish the companion
work on double stars, for which he had accumulated a vast store of
materials.[130] He died at Collingwood in Kent, May 11, 1871, in the
eightieth year of his age, and was buried in Westminster Abbey, close
beside the grave of Sir Isaac Newton.
The consideration of Sir John Herschel's Cape observations brings us to
the close of the period we are just now engaged in studying. They were
given to the world, as already stated, three years before the middle of
the century, and accurately represent the condition of sidereal science
at that date. Looking back over the fifty years traversed, we can see at
a glance how great was the stride made in the interval. Not alone was
acquaintance with individual members of the cosmos vastly extended, but
their mutual relations, the laws governing their movements, their
distances from the earth, masses, and intrinsic lustre, had begun to be
successfully investigated. _Begun to be_; for only regarding a scarcely
perceptible minority had even approximate conclusions been arrived at.
Nevertheless the whole progress of the future lay in that beginning; it
was the thin end of the wedge of exact knowledge. The principle of
measurement had been substituted for that of probability; a basis had
been found large and strong enough to enable calculation to ascend from
it to the sidereal heavens; and refinements had been introduced,
fruitful in performance, but still more in promise. Thus, rather the
kind than the amount of information collected was significant for the
time to come--rather the methods employed than the results actually
secured rendered the first half of the nineteenth century of epochal
importance in the history of our knowledge of the stars.
FOOTNOTES:
[Footnote 58: Bessel, _Populäre Vorlesungen_, pp. 6, 408.]
[Footnote 59: Fitted to the old transit instrument, July 11, 1772.]
[Footnote 60: _Briefwechsel mit Olbers_, p. xvi.]
[Footnote 61: R. Wolf, _Gesch. der Astron._, p. 518.]
[Footnote 62: Bessel, _Pop. Vorl._, p. 22.]
[Footnote 63: A new reduction of the observations upon which they were
founded was undertaken in 1896 by Herman S. Davis, of the U.S. Coast
Survey.]
[Footnote 64: Bessel, _Pop. Vorl._, p. 440.]
[Footnote 65: Durège, _Bessel's Leben und Wirken_, p. 28.]
[Footnote 66: _Bonner Beobachtungen_, Bd. iii.-v., 1859-62.]
[Footnote 67: Bessel, _Pop. Vorl._, p. 238.]
[Footnote 68: The heads of the screws applied to move the halves of the
object-glass in the Königsberg heliometer are of so considerable a size
that a thousandth part of a revolution, equivalent to 1/20 of a second
of arc, can be measured with the utmost accuracy. Main, _R. A. S. Mem._,
vol. xii., p. 53.]
[Footnote 69: _Specola Astronomica di Palermo_, lib. vi., p. 10,
_note_.]
[Footnote 70: _Monatliche Correspondenz_, vol. xxvi., p. 162.]
[Footnote 71: _Astronomische Nachrichten_, Nos. 365-366. It should be
explained that what is called the "annual parallax" of a star is only
half its apparent displacement. In other words, it is the angle
subtended at the distance of that particular star by the _radius_ of the
earth's orbit.]
[Footnote 72: _Astr. Nach._, Nos. 401-402.]
[Footnote 73: Sir R. Ball's measurements at Dunsink gave to 61 Cygni a
parallax of 0·47"; Professor Pritchard obtained, by photographic
determinations, one of 0·43".]
[Footnote 74: _Additamentum in Mensuras Micrometricas_, p. 28.]
[Footnote 75: Elkin's corrected result (in 1897) for the parallax of
Vega is 0·082".]
[Footnote 76: _Mem. Roy. Astr. Soc._, vol. xi., p. 61.]
[Footnote 77: That numbered 21,185 in Lalande's _Hist. Cél._, found by
Argelander to have a proper motion of 4·734", and by Winnecke a parallax
of O·511". _Month. Not._, vol. xviii., p. 289.]
[Footnote 78: _Fund. Astr._, p. 309.]
[Footnote 79: _Mém. Prés. à l'Ac. de St. Pétersb._, t. iii.]
[Footnote 80: _Phil. Trans._, vol. cxxxvii., p. 79.]
[Footnote 81: _Mem. Roy. Astr. Soc._, vols. xxviii. and xxxii.]
[Footnote 82: _Ibid._, vol. xlvii., p. 327.]
[Footnote 83: _Mémoires de St. Pétersbourg_, t. xxxv., No. 3, 1887;
revised in _Astr. Nach._, Nos. 3,729-30, 1901.]
[Footnote 84: _Astronomical Journal_, Nos. 213, 501.]
[Footnote 85: _Astr. Nach._, Nos. 2,999, 3,000.]
[Footnote 86: _Veröffentlichungen der Grossh. Sternwarte zu Karlsruhe_,
Bd. iv., 1892.]
[Footnote 87: _Proceedings Amsterdam Acad. of Sciences_, Jan. 27, 1900.]
[Footnote 88: _Astr. Jour._, No. 457.]
[Footnote 89: _Ibid._, Nos. 276, 497.]
[Footnote 90: _Phil. Trans._, vol. xcvi., p. 230.]
[Footnote 91: _Mém. Prés. à l'Ac. de St. Pétersbourg_, t. iii., p. 603
(read Feb. 5, 1837).]
[Footnote 92: _Die Centralsonne, Astr. Nach._, Nos. 566-567, 1846.]
[Footnote 93: Sir J. Herschel, note to _Treatise on Astronomy_, and
_Phil. Trans._, vol. cxxiii., part ii., p. 502.]
[Footnote 94: The position is (as Sir J. Herschel pointed out, _Outlines
of Astronomy_, p. 631, 10th ed.) placed beyond the range of reasonable
probability by its remoteness (fully 26°) from the galactic plane.]
[Footnote 95: Mädler in _Westermann's Jahrbuch_, 1867, p. 615.]
[Footnote 96: Letter from Bessel to Sir J. Herschel, _Month. Not._, vol.
vi., p. 139.]
[Footnote 97: Wolf, _Gesch. d. Astr._, p. 743, _note_.]
[Footnote 98: _Astr. Nach._, Nos. 745-748.]
[Footnote 99: _Astr. Jour._, No. 440.]
[Footnote 100: Adopting Elkin's revised parallax for Procyon of 0·325".]
[Footnote 101: _Astr. Nach._, Nos. 1371-1373.]
[Footnote 102: _Ueber die Doppelsterne_, Bericht, 1827, p. 22.]
[Footnote 103: _Ueber die Doppelsterne_, Bericht, 1827, p. 25.]
[Footnote 104: _Mensuræ Micr._, p. xcix.]
[Footnote 105: _Stellarum Fixarum imprimis Duplicium et Multiplicum
Positiones Mediæ_, pp. cxc., cciii.]
[Footnote 106: For instance, the southern stars, 36A Ophiuchi (itself
double) and 30 Scorpii, which are 12' 10" apart. _Ibid._, p. cciii.]
[Footnote 107: _Stellarum Fixarum_, etc., p. ccliii.]
[Footnote 108: _Études d'Astronomie Stellaire_, 1847, p. 82.]
[Footnote 109: _Ibid._, p. 86.]
[Footnote 110: See Encke's criticism in _Astr. Nach._, No. 622.]
[Footnote 111: _Phil. Trans._, vol. cxiv., part iii., 1824.]
[Footnote 112: _Conn. d. Temps_, 1830.]
[Footnote 113: _R. A. S. Mem._, vol. v., p. 178, 1833.]
[Footnote 114: _Astr. and Astrophysics_, vol. xii., p. 581.]
[Footnote 115: _Popular Astr._, vol. i., p. 243.]
[Footnote 116: _Phil. Trans._, vol. cxxiii., and _Results_, etc.,
Introd.]
[Footnote 117: _Results of Astronomical Observations made during the
years 1834-8 at the Cape of Good Hope._]
[Footnote 118: _Results_, etc., p. 147.]
[Footnote 119: See Proctor's _Universe of Stars_, p. 92.]
[Footnote 120: _A Treatise on Astronomy_, 1833, p. 406.]
[Footnote 121: _Results_, etc., p. 139.]
[Footnote 122: _Ibid._, pp. 24, 142.]
[Footnote 123: _Phil. Trans._, vol. cxxiii., p. 503.]
[Footnote 124: _Results_, etc., p. 136.]
[Footnote 125: Loomis, _Month. Not._, vol. xxix., p. 298.]
[Footnote 126: See the Author's _System of the Stars_, pp. 116-120.]
[Footnote 127: _Outlines of Astr._, App. I.]
[Footnote 128: _Phil. Trans._, vol. cxix., p. 27.]
[Footnote 129: Dr. Dreyer's New General Catalogue, published in 1888 as
vol. xlix. of the Royal Astronomical Society's _Memoirs_, is an
enlargement of Herschel's work. It includes 7,840 entries, and was
supplemented, in 1895, by an "Index Catalogue" of 1,529 nebulæ
discovered 1888 to 1894. _Mem. R. A. S._, vol. li.]
[Footnote 130: A list of 10,320 composite stars was drawn out by him in
order of right ascension, and has been published in vol. xl. of _Mem. R.
A. S._; but the data requisite for their formation into a catalogue were
not forthcoming. See Main's and Pritchard's _Preface_ to above, and
Dunkin's _Obituary Notices_, p. 73.]
CHAPTER III
_PROGRESS OF KNOWLEDGE REGARDING THE SUN_
The discovery of sun-spots in 1610 by Fabricius and Galileo first opened
a way for inquiry into the solar constitution; but it was long before
that way was followed with system or profit. The seeming irregularity of
the phenomena discouraged continuous attention; casual observations were
made the basis of arbitrary conjectures, and real knowledge received
little or no increase. In 1620 we find Jean Tarde, Canon of Sarlat,
arguing that because the sun is "the eye of the world," and the eye of
the world _cannot suffer from ophthalmia_, therefore the appearances in
question must be due, not to actual specks or stains on the bright solar
disc, but to the transits of a number of small planets across it! To
this new group of heavenly bodies he gave the name of "Borbonia Sidera,"
and they were claimed in 1633 for the House of Hapsburg, under the title
of "Austriaca Sidera" by Father Malapertius, a Belgian Jesuit.[131] A
similar view was temporarily maintained against Galileo by the justly
celebrated Father Scheiner of Ingolstadt, and later by William
Gascoigne, the inventor of the micrometer; but most of those who were
capable of thinking at all on such subjects (and they were but few)
adhered either to the _cloud theory_ or to the _slag theory_ of
sun-spots. The first was championed by Galileo, the second by Simon
Marius, "astronomer and physician" to the brother Margraves of
Brandenburg. The latter opinion received a further notable development
from the fact that in 1618, a year remarkable for the appearance of
three bright comets, the sun was almost free from spots; whence it was
inferred that the cindery refuse from the great solar conflagration,
which usually appeared as dark blotches on its surface, was occasionally
thrown off in the form of comets, leaving the sun, like a snuffed taper,
to blaze with renewed brilliancy.[132]
In the following century, Derham gathered from observations carried on
during the years 1703-11, "That the spots on the sun are caused by the
eruption of some new volcano therein, which at first pouring out a
prodigious quantity of smoke and other opacous matter, causeth the
spots; and as that fuliginous matter decayeth and spendeth itself, and
the volcano at last becomes more torrid and flaming, so the spots decay,
and grow to umbræ, and at last to faculæ."[133]
The view, confidently upheld by Lalande,[134] that spots were rocky
elevations uncovered by the casual ebbing of a luminous ocean, the
surrounding penumbræ representing shoals or sandbanks, had even less to
recommend it than Derham's volcanic theory. Both were, however,
significant of a growing tendency to bring solar phenomena within the
compass of terrestrial analogies.
For 164 years, then, after Galileo first levelled his telescope at the
setting sun, next to nothing was learned as to its nature; and the facts
immediately ascertained, of its rotation on an axis nearly erect to the
plane of the ecliptic, in a period of between twenty-five and twenty-six
days, and of the virtual limitation of the spots to a so-called "royal"
zone extending some thirty degrees north and south of the solar equator,
gained little either in precision or development from five generations
of astronomers.
But in November, 1769, a spot of extraordinary size engaged the
attention of Alexander Wilson, professor of astronomy in the University
of Glasgow. He watched it day by day, and to good purpose. As the great
globe slowly revolved, carrying the spot towards its western edge, he
was struck with the gradual contraction and final disappearance of the
penumbra _on the side next the centre of the disc_; and when on the 6th
of December the same spot re-emerged on the eastern limb, he perceived,
as he had anticipated, that the shady zone was now deficient _on the
opposite side_, and resumed its original completeness as it returned to
a central position. In other spots subsequently examined by him, similar
perspective effects were visible, and he proved in 1774,[135] by strict
geometrical reasoning, that they could only arise in vast photospheric
excavations. It was not, indeed, the first time that such a view had
been suggested. Father Scheiner's later observations plainly
foreshadowed it;[136] a conjecture to the same effect was emitted by
Leonard Rost of Nuremburg early in the eighteenth century;[137] both by
Lahire in 1703 and by J. Cassini in 1719 spots had been seen as notches
on the solar limb; while in 1770 Pastor Schülen of Essingen, from the
careful study of phenomena similar to those noted by Wilson, concluded
their depressed nature.[138] Modern observations, nevertheless, prove
those phenomena to be by no means universally present.
Wilson's general theory of the sun was avowedly tentative. It took the
modest form of an interrogatory. "Is it not reasonable to think," he
asks, "that the great and stupendous body of the sun is made up of two
kinds of matter, very different in their qualities; that by far the
greater part is solid and dark, and that this immense and dark globe is
encompassed with a thin covering of that resplendent substance from
which the sun would seem to derive the whole of his vivifying heat and
energy?"[139] He further suggests that the excavations or spots may be
occasioned "by the working of some sort of elastic vapour which is
generated within the dark globe," and that the luminous matter, being in
some degree fluid, and being acted upon by gravity, tends to flow down
and cover the nucleus. From these hints, supplemented by his own
diligent observations and sagacious reasonings, Herschel elaborated a
scheme of solar constitution which held its ground until the physics of
the sun were revolutionised by the spectroscope.
A cool, dark, solid globe, its surface diversified with mountains and
valleys, clothed in luxuriant vegetation, and "richly stored with
inhabitants," protected by a heavy cloud-canopy from the intolerable
glare of the upper luminous region, where the dazzling coruscations of a
solar aurora some thousands of miles in depth evolved the stores of
light and heat which vivify our world--such was the central luminary
which Herschel constructed with his wonted ingenuity, and described with
his wonted eloquence.
"This way of considering the sun and its atmosphere," he says,[140]
"removes the great dissimilarity we have hitherto been used to find
between its condition and that of the rest of the great bodies of the
solar system. The sun, viewed in this light, appears to be nothing else
than a very eminent, large, and lucid planet, evidently the first, or,
in strictness of speaking, the only primary one of our system; all
others being truly secondary to it. Its similarity to the other globes
of the solar system with regard to its solidity, its atmosphere, and its
diversified surface, the rotation upon its axis, and the fall of heavy
bodies, leads us on to suppose that it is most probably also inhabited,
like the rest of the planets, by beings whose organs are adapted to the
peculiar circumstances of that vast globe."
We smile at conclusions which our present knowledge condemns as
extravagant and impossible, but such incidental flights of fancy in no
way derogate from the high value of Herschel's contributions to solar
science. The cloud-like character which he attributed to the radiant
shell of the sun (first named by Schröter the "photosphere") is borne
out by all recent investigations; he observed its mottled or corrugated
aspect, resembling, as he described it, the roughness on the rind of an
orange; showed that "faculæ" are elevations or heaped-up ridges of the
disturbed photospheric matter; and threw out the idea that spots may
ensue from an excess of the ordinary luminous emissions. A certain
"empyreal" gas was, he supposed (very much as Wilson had done),
generated in the body of the sun, and rising everywhere by reason of its
lightness, made for itself, when in moderate quantities, small openings
or "pores,"[141] abundantly visible as dark points on the solar disc.
But should an uncommon quantity be formed, "it will," he maintained,
"burst through the planetary[142] regions of clouds, and thus will
produce great openings; then, spreading itself above them, it will
occasion large shallows (penumbræ), and mixing afterwards gradually with
other superior gases, it will promote the increase, and assist in the
maintenance, of the general luminous phenomena."[143]
This partial anticipation of the modern view that the solar radiations
are maintained by some process of circulation within the solar mass, was
reached by Herschel through prolonged study of the phenomena in
question. The novel and important idea contained in it, however, it was
at that time premature to attempt to develop. But though many of the
subtler suggestions of Herschel's genius passed unnoticed by his
contemporaries, the main result of his solar researches was an
unmistakable one. It was nothing less than the definitive introduction
into astronomy of the paradoxical conception of the central fire and
hearth of our system as a cold, dark, terrestrial mass, wrapt in a
mantle of innocuous radiance--an earth, so to speak, within--a sun
without.
Let us pause for a moment to consider the value of this remarkable
innovation. It certainly was not a step in the direction of truth. On
the contrary, the crude notions of Anaxagoras and Xeno approached more
nearly to what we now know of the sun, than the complicated structure
devised for the happiness of a nobler race of beings than our own by the
benevolence of eighteenth-century astronomers. And yet it undoubtedly
constituted a very important advance in science. It was the first
earnest attempt to bring solar phenomena within the compass of a
rational system; to put together into a consistent whole the facts
ascertained; to fabricate, in short, a solar machine that would in some
fashion work. It is true that the materials were inadequate and the
design faulty. The resulting construction has not proved strong enough
to stand the wear and tear of time and discovery, but has had to be
taken to pieces and remodelled on a totally different plan. But the work
was not therefore done in vain. None of Bacon's aphorisms show a clearer
insight into the relations between the human mind and the external world
than that which declares "Truth to emerge sooner from error than from
confusion."[144] A definite theory (even if a false one) gives
holding-ground to thought. Facts acquire a meaning with reference to it.
It affords a motive for accumulating them and a means of co-ordinating
them; it provides a framework for their arrangement, and a receptacle
for their preservation, until they become too strong and numerous to be
any longer included within arbitrary limits, and shatter the vessel
originally framed to contain them.
Such was the purpose subserved by Herschel's theory of the sun. It
helped to _clarify_ ideas on the subject. The turbid sense of groping
and viewless ignorance gave place to the lucidity of a possible scheme.
The persuasion of knowledge is a keen incentive to its increase. Few men
care to investigate what they are obliged to admit themselves entirely
ignorant of; but once started on the road of knowledge, real or
supposed, they are eager to pursue it. By the promulgation of a
confident and consistent view regarding the nature of the sun,
accordingly, research was encouraged, because it was rendered hopeful,
and inquirers were shown a path leading indefinitely onwards where an
impassable thicket had before seemed to bar the way.
We have called the "terrestrial" theory of the sun's nature an
innovation, and so, as far as its general acceptance is concerned, it
may justly be termed; but, like all successful innovations, it was a
long time brewing. It is extremely curious to find that Herschel had a
predecessor in its advocacy who never looked through a telescope (nor,
indeed, imagined the possibility of such an instrument), who knew
nothing of sun-spots, was still (mistaken assertions to the contrary
notwithstanding) in the bondage of the geocentric system, and regarded
nature from the lofty standpoint of an idealist philosophy. This was the
learned and enlightened Cardinal Cusa, a fisherman's son from the banks
of the Moselle, whose distinguished career in the Church and in
literature extended over a considerable part of the fifteenth century
(1401-64). In his singular treatise _De Doctâ Ignorantiâ_, one of the
most notable literary monuments of the early Renaissance, the following
passage occurs:--"To a spectator on the surface of the sun, the
splendour which appears to us would be invisible, since it contains, as
it were, an earth for its central mass, with a circumferential envelope
of light and heat, and between the two an atmosphere of water and clouds
and translucent air." The luminary of Herschel's fancy could scarcely be
more clearly portrayed; some added words, however, betray the origin of
the Cardinal's idea. "The earth also," he says, "would appear as a
shining star to any one outside the fiery element." It was, in fact, an
extension to the sun of the ancient elemental doctrine; but an extension
remarkable at that period, as premonitory of the tendency, so powerfully
developed by subsequent discoveries, to assimilate the orbs of heaven to
the model of our insignificant planet, and to extend the brotherhood of
our system and our species to the farthest limit of the visible or
imaginable universe.
In later times we find Flamsteed communicating to Newton, March 7, 1681,
his opinion "that the substance of the sun is terrestrial matter, his
light but the liquid menstruum encompassing him."[145] Bode in 1776
arrived independently at the conclusion that "the sun is neither burning
nor glowing, but in its essence a dark planetary body, composed like our
earth of land and water, varied by mountains and valleys, and enveloped
in a vaporous atmosphere";[146] and the learned in general applauded and
acquiesced. The view, however, was in 1787 still so far from popular,
that the holding of it was alleged as a proof of insanity in Dr. Elliot
when accused of a murderous assault on Miss Boydell. His friend Dr.
Simmons stated on his behalf that he had received from him in the
preceding January a letter giving evidence of a deranged mind, wherein
he asserted "that the sun is not a body of fire, as hath been hitherto
supposed, but that its light proceeds from a dense and universal aurora,
which may afford ample light to the inhabitants of the surface beneath,
and yet be at such a distance aloft as not to annoy them. No objection,
he saith, ariseth to that great luminary's being inhabited; vegetation
may obtain there as well as with us. There may be water and dry land,
hills and dales, rain and fair weather; and as the light, so the season
must be eternal, consequently it may easily be conceived to be by far
the most blissful habitation of the whole system!" The Recorder,
nevertheless, objected that if an extravagant hypothesis were to be
adduced as proof of insanity, the same might hold good with regard to
some other speculators, and desired Dr. Simmons to tell the court what
he thought of the theories of Burnet and Buffon.[147]
Eight years later, this same "extravagant hypothesis," backed by the
powerful recommendation of Sir William Herschel, obtained admittance to
the venerable halls of science, there to abide undisturbed for nearly
seven decades. Individual objectors, it is true, made themselves heard,
but their arguments had little effect on the general body of opinion.
Ruder blows were required to shatter an hypothesis flattering to human
pride of invention in its completeness, in the plausible detail of
observations by which it seemed to be supported, and in its
condescension to the natural pleasure in discovering resemblance under
all but total dissimilarity.
Sir John Herschel included among the results of his multifarious labours
at the Cape of Good Hope a careful study of the sun-spots conspicuously
visible towards the end of the year 1836 and in the early part of 1837.
They were remarkable, he tells us, for their forms and arrangement, as
well as for their number and size; one group, measured on the 29th of
March in the latter year, covering (apart from what may be called its
outlying dependencies) the vast area of five square minutes or 3,780
million square miles.[148] We have at present to consider, however, not
so much these observations in themselves, as the chain of theoretical
suggestions by which they were connected. The distribution of spots, it
was pointed out, on two zones parallel to the equator, showed plainly
their intimate connection with the solar rotation, and indicated as
their cause fluid circulations analogous to those producing the
terrestrial trade and anti-trade winds.
"The spots, in this view of the subject," he went on to say,[149] "would
come to be assimilated to those regions on the earth's surface where,
for the moment, hurricanes and tornadoes prevail; the upper stratum
being temporarily carried downwards, displacing by its impetus the two
strata of luminous matter beneath, the upper of course to a greater
extent than the lower, and thus wholly or partially denuding the opaque
surface of the sun below. Such processes cannot be unaccompanied by
vorticose motions, which, left to themselves, die away by degrees and
dissipate, with the peculiarity that their lower portions come to rest
more speedily than their upper, by reason of the greater resistance
below, as well as the remoteness from the point of action, which lies in
a higher region, so that their centres (as seen in our waterspouts,
which are nothing but small tornadoes) appear to retreat upwards. Now
this agrees perfectly with what is observed during the obliteration of
the solar spots, which appear as if filled in by the collapse of their
sides, the penumbra closing in upon the spot and disappearing after it."
But when it comes to be asked whether a cause can be found by which a
diversity of solar temperature might be produced corresponding with that
which sets the currents of the terrestrial atmosphere in motion, we are
forced to reply that we know of no such cause. For Sir John Herschel's
hypothesis of an increased retention of heat at the sun's equator, due
to the slightly spheroidal or bulging form of its outer atmospheric
envelope, assuredly gives no sufficient account of such circulatory
movements as he supposed to exist. Nevertheless, the view that the sun's
rotation is intimately connected with the formation of spots is so
obviously correct, that we can only wonder it was not thought of sooner,
while we are even now unable to explain with any certainty _how_ it is
so connected.
Mere scrutiny of the solar surface, however, is not the only means of
solar observation. We have a satellite, and that satellite from time to
time acts most opportunely as a screen, cutting off a part or the whole
of those dazzling rays in which the master-orb of our system veils
himself from over-curious regards. The importance of eclipses to the
study of the solar surroundings is of comparatively recent recognition;
nevertheless, much of what we know concerning them has been snatched, as
it were, by surprise under favour of the moon. In former times, the sole
astronomical use of such incidents was the correction of the received
theories of the solar and lunar movements; the precise time of their
occurrence was the main fact to be noted, and subsidiary phenomena
received but casual attention. Now, their significance as a geometrical
test of tabular accuracy is altogether overshadowed by the interest
attaching to the physical observations for which they afford propitious
occasions. This change may be said to date, in its pronounced form, from
the great eclipse of 1842. Although a necessary consequence of the
general direction taken by scientific progress, it remains associated in
a special manner with the name of Francis Baily.
The "philosopher of Newbury" was by profession a London stockbroker, and
a highly successful one. Nevertheless, his services to science were
numerous and invaluable, though not of the brilliant kind which attract
popular notice. Born at Newbury in Berkshire, April 28, 1774, and placed
in the City at the age of fourteen, he derived from the acquaintance of
Dr. Priestley a love of science which never afterwards left him. It was,
however, no passion such as flames up in the brain of the destined
discoverer, but a regulated inclination, kept well within the bounds of
an actively pursued commercial career. After travelling for a year or
two in what were then the wilds of North America, he went on the Stock
Exchange in 1799, and earned during twenty-four years of assiduous
application to affairs a high reputation for integrity and ability, to
which corresponded an ample fortune. In the meantime the Astronomical
Society (largely through his co-operation) had been founded; he had for
three years acted as its secretary, and he now felt entitled to devote
himself exclusively to a subject which had long occupied his leisure
hours. He accordingly in 1825 retired from business, purchased a house
in Tavistock Place, and fitted up there a small observatory. He was,
however, by preference a computator rather than an observer. What Sir
John Herschel calls the "archæology of practical astronomy" found in him
an especially zealous student. He re-edited the star-catalogues of
Ptolemy, Ulugh Beigh, Tycho Brahe, Hevelius, Halley, Flamsteed,
Lacaille, and Mayer; calculated the eclipse of Thales and the eclipse of
Agathocles, and vindicated the memory of the first Astronomer Royal. But
he was no less active in meeting present needs than in revising past
performances. The subject of the reduction of observations, then, as we
have already explained,[150] in a state of deplorable confusion,
attracted his most earnest attention, and he was close on the track of
Bessel when made acquainted with the method of simplification devised at
Königsberg. Anticipated as an inventor, he could still be of eminent use
as a promoter of these valuable improvements; and, carrying them out on
a large scale in the star-catalogue of the Astronomical Society
(published in 1827), "he put" (in the words of Herschel) "the
astronomical world in possession of a power which may be said, without
exaggeration, to have changed the face of sidereal astronomy."[151]
His reputation was still further enhanced by his renewal, with vastly
improved apparatus, of the method, first used by Henry Cavendish in
1797-98, for determining the density of the earth. From a series of no
less than 2,153 delicate and difficult experiments, conducted at
Tavistock Place during the years 1838-42, he concluded our planet to
weigh 5·66 as much as a globe of water of the same bulk; and this result
slightly corrected is still accepted as a very close approximation of
the truth.
What we have thus glanced at is but a fragment of the truly surprising
mass of work accomplished by Baily in the course of a variously occupied
life. A rare combination of qualities fitted him for his task. Unvarying
health, undisturbed equanimity, methodical habits, the power of directed
and sustained thought, combined to form in him an intellectual toiler of
the surest, though not perhaps of the highest quality. He was in harness
almost to the end. He was destined scarcely to know the miseries of
enforced idleness or of consciously failing powers. In 1842 he completed
the laborious reduction of Lalande's great catalogue, undertaken at the
request of the British Association, and was still engaged in seeing it
through the press when he was attacked with what proved his last, as it
was probably his first serious illness. He, however, recovered
sufficiently to attend the Oxford Commemoration of July 2, 1844, where
an honorary degree of D.C.L. was conferred upon him in company with Airy
and Struve; but sank rapidly after the effort, and died on the 30th of
August following, at the age of seventy, lamented and esteemed by all
who knew him.
It is now time to consider his share in the promotion of solar research.
Eclipses of the sun, both ancient and modern, were a speciality with
him, and he was fortunate in those which came under his observation.
Such phenomena are of three kinds--partial, annular, and total. In a
partial eclipse, the moon, instead of passing directly between us and
the sun, slips by, as it were, a little on one side, thus cutting off
from our sight only a portion of his surface. An annular eclipse, on the
other hand, takes place when the moon is indeed centrally interposed,
but falls short of the apparent size required for the entire concealment
of the solar disc, which consequently remains visible as a bright ring
or annulus, even when the obscuration is at its height. In a total
eclipse, on the contrary, the sun completely disappears behind the dark
body of the moon. The difference of the two latter varieties is due to
the fact that the apparent diameter of the sun and moon are so nearly
equal as to gain alternate preponderance one over the other through the
slight periodical changes in their respective distances from the earth.
Now, on the 15th of May, 1836, an annular eclipse was visible in the
northern parts of Great Britain, and was observed by Baily at Inch
Bonney, near Jedburgh. It was here that he saw the phenomenon which
obtained the name of "Baily's Beads," from the notoriety conferred upon
it by his vivid description.
"When the cusps of the sun," he writes, "were about 40° asunder, a row
of lucid points, like a string of bright beads, irregular in size and
distance from each other, _suddenly_ formed round that part of the
circumference of the moon that was about to enter on the sun's disc. Its
formation, indeed, was so rapid that it presented the appearance of
having been caused by the ignition of a fine train of gunpowder.
Finally, as the moon pursued her course, the dark intervening spaces
(which, at their origin, had the appearance of lunar mountains in high
relief, and which still continued attached to the sun's border) were
stretched out into long, black, thick, parallel lines, joining the limbs
of the sun and moon; when all at once they _suddenly_ gave way, and left
the circumference of the sun and moon in those points, as in the rest,
comparatively smooth and circular, and the moon perceptibly advanced on
the face of the sun."[152]
These curious appearances were not an absolute novelty. Weber in 1791,
and Von Zach in 1820, had seen the "beads"; Van Swinden had described
the "belts" or "threads."[153] These last were, moreover (as Baily
clearly perceived), completely analogous to the "black ligament" which
formed so troublesome a feature in the transits of Venus in 1764 and
1769, and which, to the regret and confusion, though no longer to the
surprise of observers, was renewed in that of 1874. The phenomenon is
largely an effect of what is called _irradiation_, by which a bright
object seems to encroach upon a dark one; but under good atmospheric and
instrumental conditions it becomes inconspicuous. The "Beads" must
always appear when the projected lunar edge is serrated with mountains.
In Baily's observation, they were exaggerated and distorted by an
irradiative _clinging together_ of the limbs of sun and moon.
The immediate result, however, was powerfully to stimulate attention to
solar eclipses in their _physical_ aspect. Never before had an
occurrence of the kind been expected so eagerly or prepared for so
actively as that which was total over Central and Southern Europe on the
8th of July, 1842. Astronomers hastened from all quarters to the
favoured region. The Astronomer Royal (Airy) repaired to Turin; Baily to
Pavia; Otto Struve threw aside his work amidst the stars at Pulkowa, and
went south as far as Lipeszk; Schumacher travelled from Altona to
Vienna; Arago from Paris to Perpignan. Nor did their trouble go
unrewarded. The expectations of the most sanguine were outdone by the
wonders disclosed.
Baily (to whose narrative we again have recourse) had set up his
Dollond's achromatic in an upper room of the University of Pavia, and
was eagerly engaged in noting a partial repetition of the singular
appearances seen by him in 1836, when he was "astounded by a tremendous
burst of applause from the streets below, and at the same moment was
electrified at the sight of one of the most brilliant and splendid
phenomena that can well be imagined. For at that instant the dark body
of the moon was suddenly surrounded with a corona, or kind of bright
glory similar in shape and relative magnitude to that which painters
draw round the heads of saints, and which by the French is designated an
_auréole_. Pavia contains many thousand inhabitants, the major part of
whom were, at this early hour, walking about the streets and squares or
looking out of windows, in order to witness this long-talked-of
phenomenon; and when the total obscuration took place, which was
_instantaneous_, there was a universal shout from every observer, which
'made the welkin ring,' and, for the moment, withdrew my attention from
the object with which I was immediately occupied. I had indeed
anticipated the appearance of a luminous circle round the moon during
the time of total obscurity; but I did not expect, from any of the
accounts of preceding eclipses that I had read, to witness so
magnificent an exhibition as that which took place.... The breadth of
the corona, measured from the circumference of the moon, appeared to me
to be nearly equal to half the moon's diameter. It had the appearance of
brilliant rays. The light was most dense close to the border of the
moon, and became gradually and uniformly more attenuate as its distance
therefrom increased, assuming the form of diverging rays in a
rectilinear line, which at the extremity were more divided, and of an
unequal length; so that in no part of the corona could I discover the
regular and well-defined shape of a ring at its _outer_ margin. It
appeared to me to have the sun for its centre, but I had no means of
taking any accurate measures for determining this point. Its colour was
quite white, not pearl-colour, nor yellow, nor red, and the rays had a
vivid and flickering appearance, somewhat like that which a gaslight
illumination might be supposed to assume if formed into a similar
shape.... Splendid and astonishing, however, as this remarkable
phenomenon really was, and although it could not fail to call forth the
admiration and applause of every beholder, yet I must confess that there
was at the same time something in its singular and wonderful appearance
that was appalling; and I can readily imagine that uncivilised nations
may occasionally have become alarmed and terrified at such an object,
more especially at times when the true cause of the occurrence may have
been but faintly understood, and the phenomenon itself wholly
unexpected.
"But the most remarkable circumstance attending the phenomenon was the
appearance of _three large protuberances_ apparently emanating from the
circumference of the moon, but evidently forming a portion of the
corona. They had the appearance of mountains of a prodigious elevation;
their colour was red, tinged with lilac or purple; perhaps the colour of
the peach-blossom would more nearly represent it. They somewhat
resembled the snowy tops of the Alpine mountains when coloured by the
rising or setting sun. They resembled the Alpine mountains also in
another respect, inasmuch as their light was perfectly steady, and had
none of that flickering or sparkling motion so visible in other parts of
the corona. All the three projections were of the same roseate cast of
colour, and very different from the brilliant vivid white light that
formed the corona; but they differed from each other in magnitude....
The whole of these three protuberances were visible even to the last
moment of total obscuration; at least, I never lost sight of them when
looking in that direction; and when the first ray of light was admitted
from the sun, they vanished, with the corona, altogether, and daylight
was instantaneously restored."[154]
Notwithstanding unfavourable weather, the "red flames" were perceived
with little less clearness and no less amazement from the Superga than
at Pavia, and were even discerned by Mr. Airy with the naked eye. "Their
form" (the Astronomer Royal wrote) "was nearly that of saw-teeth in the
position proper for a circular saw turned round in the same direction in
which the hands of a watch turn.... Their colour was a full lake-red,
and their brilliancy greater than that of any other part of the
ring."[155]
The height of these extraordinary objects was estimated by Arago at two
minutes of arc, representing, at the sun's distance, an actual elevation
of 54,000 miles. When carefully watched, the rose-flush of their
illumination was perceived to fade through violet to white as the light
returned, the same changes in a reversed order having accompanied their
first appearance. Their forms, however, during about three minutes of
visibility, showed no change, although of so apparently unstable a
character as to suggest to Arago "mountains on the point of crumbling
into ruins" through topheaviness.[156]
The corona, both as to figure and extent, presented very different
appearances at different stations. This was no doubt due to varieties in
atmospheric conditions. At the Superga, for instance, all details of
structure seem to have been effaced by the murky air, only a
comparatively feeble ring of light being seen to encircle the moon.
Elsewhere, a brilliant radiated formation was conspicuous, spreading at
four opposite points into four vast luminous expansions, compared to
feather-plumes or _aigrettes_.[157] Arago at Perpignan noticed
considerable irregularities in the divergent rays. Some appeared curved
and twisted, a few lay _across_ the others, in a direction almost
tangential to the moon's limb, the general effect being described as
that of a "hank of thread in disorder."[158] At Lipeszk, where the sun
stood much higher above the horizon than in Italy or France, the corona
showed with surprising splendour. Its apparent extent was judged by
Struve to be no less than twenty-five minutes (more than six times
Airy's estimate), while the great plumes spread their radiance to three
or four degrees from the dark lunar edge. So dazzling was the light that
many well-instructed persons denied the totality of the eclipse. Nor was
the error without precedent, although the appearances attending
respectively a total and an annular eclipse are in reality wholly
dissimilar. In the latter case, the surviving ring of sunlight becomes
so much enlarged by irradiation, that the interposed dark lunar body is
reduced to comparative insignificance, or even invisibility. Maclaurin
tells us[159] that during an eclipse of this character which he observed
at Edinburgh in 1737, "gentlemen by no means shortsighted declared
themselves unable to discern the moon upon the sun without the aid of a
smoked glass;" and Baily (who, however, _was_ shortsighted) could
distinguish, in 1836, with the naked eye, no trace of "the globe of
purple velvet" which the telescope revealed as projected upon the face
of the sun.[160] Moreover, the diminution of light is described by him
as "little more than might be caused by a temporary cloud passing over
the sun"; the birds continued in full song, and "one cock in particular
was crowing with all his might while the annulus was forming."
Very different were the effects of the eclipse of 1842, as to which some
interesting particulars were collected by Arago.[161] Beasts of burthen,
he tells us, paused in their labour, and could by no amount of
punishment be induced to move until the sun reappeared. Birds and beasts
abandoned their food; linnets were found dead in their cages; even ants
suspended their toil. Diligence-horses, on the other hand, seemed as
insensible to the phenomenon as locomotives. The convolvulus and some
other plants closed their leaves, but those of the mimosa remained open.
The little light that remained was of a livid hue. One observer
described the general coloration as resembling the lees of wine, but
human faces showed pale olive or greenish. We may, then, rest assured
that none of the remarkable obscurations recorded in history were due to
eclipses of the annular kind.
The existence of the corona is no modern discovery. Indeed, it is too
conspicuous an apparition to escape notice from the least attentive or
least practised observer of a total eclipse. Nevertheless, explicit
references to it are rare in early times. Plutarch, however, speaks of a
"certain splendour" compassing round the hidden edge of the sun, as a
regular feature of total eclipses;[162] and the corona is expressly
mentioned in a description of an eclipse visible at Corfu in 968
A.D.[163] The first to take the phenomenon into scientific consideration
was Kepler. He showed, from the orbital positions at the time of the sun
and moon, that an eclipse observed by Clavius at Rome in 1567 could not
have been annular,[164] as the dazzling coronal radiance visible during
the obscuration had caused it to be believed. Although he himself never
witnessed a total eclipse of the sun, he carefully collected and
compared the remarks of those more fortunate, and concluded that the
ring of "flame-like splendour" seen on such occasions was caused by the
reflection of the solar rays from matter condensed in the neighbourhood
either of the sun or moon.[165] To the solar explanation he gave his own
decided preference; but, with one of those curious flashes of
half-prophetic insight characteristic of his genius, declared that "it
should be laid by ready for use, not brought into immediate
requisition."[166] So literally was his advice acted upon, that the
theory, which we now know to be (broadly speaking) the correct one, only
emerged from the repository of anticipated truths after 236 years of
almost complete retirement, and even then timorously and with
hesitation.
The first eclipse of which the attendant phenomena were observed with
tolerable exactness was that which was central in the South of France,
May 12, 1706. Cassini then put forward the view that the "crown of pale
light" seen round the lunar disc was caused by the illumination of the
zodiacal light;[167] but it failed to receive the attention which, as a
step in the right direction, it undoubtedly merited. Nine years later we
meet with Halley's comments on a similar event, the first which had
occurred in London since March 20, 1140. By nine in the morning of May
3, 1715, the obscuration, he tells us, "was about ten digits,[168] when
the face and colour of the sky began to change from perfect serene azure
blue to a more dusky livid colour, having an eye of purple intermixt....
A few seconds before the sun was all hid, there discovered itself round
the moon a luminous ring, about a digit or perhaps a tenth part of the
moon's diameter in breadth. It was of a pale whiteness, or rather pearl
colour, seeming to be a little tinged with the colours of the iris, and
to be concentric with the moon, whence I concluded it the moon's
atmosphere. But the great height thereof, far exceeding our earth's
atmosphere, and the observation of some, who found the breadth of the
ring to increase on the west side of the moon as emersion approached,
together with the contrary sentiments of those whose judgment I shall
always revere" (Newton is most probably referred to), "makes me less
confident, especially in a matter whereto I confess I gave not all the
attention requisite." He concludes by declining to decide whether the
"enlightened atmosphere," which the appearance "in all respects
resembled," "belonged to sun or moon."[169]
A French Academician, who happened to be in London at the time, was less
guarded in expressing an opinion. The Chevalier de Louville declared
emphatically for the lunar atmospheric theory of the corona,[170] and
his authority carried great weight. It was, however, much discredited by
an observation made by Maraldi in 1724, to the effect that the luminous
ring, instead of travelling _with_ the moon, was traversed _by_ it.[171]
This was in reality decisive, though, as usual, belief lagged far behind
demonstration. In 1715 a novel explanation had been offered by Delisle
and Lahire,[172] supported by experiments regarded at the time as
perfectly satisfactory. The aureola round the eclipsed sun, they argued,
is simply a result of the _diffraction_, or apparent bending of the
sunbeams that graze the surface of the lunar globe--an effect of the
same kind as the coloured fringes of shadows. And this view prevailed
amongst men of science until (and even after) Brewster showed, with
clear and simple decisiveness, that such an effect could by no
possibility be appreciable at our distance from the moon.[173] Don José
Joaquim de Ferrer, however, who observed a total eclipse of the sun at
Kinderhook, in the State of New York, on June 16, 1806, ignoring this
refined optical _rationale_, considered two alternative explanations of
the phenomenon as alone possible. The bright ring round the moon must be
due to the illumination either of a lunar or of a solar atmosphere. If
the former, he calculated that it should have a height fifty times that
of the earth's gaseous envelope. "Such an atmosphere," he rightly
concluded, "cannot belong to the moon, but must without any doubt belong
to the sun."[174] But he stood alone in this unhesitating assertion.
The importance of the problem was first brought fully home to
astronomers by the eclipse of 1842. The brilliant and complex appearance
which on that occasion challenged the attention of so many observers,
demanded and received, no longer the casual attention hitherto bestowed
upon it, but the most earnest study of those interested in the progress
of science. Nevertheless, it was only by degrees, and through a process
of "exclusions" (to use a Baconian phrase) that the corona was put in
its right place as a solar appendage. As every other available
explanation proved inadmissible and dropped out of sight, the broad
presentation of fact remained, which, though of sufficiently obvious
interpretation, was long and persistently misconstrued. Nor was it until
1869 that absolutely decisive evidence on the subject was forthcoming,
as we shall see further on.
Sir John Herschel, writing to his venerable aunt, relates that when the
brilliant red flames burst into view behind the dark moon on the morning
of the 8th of July, 1842, the populace of Milan, with the usual
inconsequence of a crowd, raised the shout, "_Es leben die
Astronomen!_"[175] In reality, none were less prepared for their
apparition than the class to whom the applause due to the magnificent
spectacle was thus adjudged. And in some measure through their own
fault, for many partial hints and some distinct statements from earlier
observers had given unheeded notice that some such phenomenon might be
expected to attend a solar eclipse.
What we now call the "chromosphere" is an envelope of glowing gases, by
which the sun is completely covered, and from which the "prominences"
are emanations, eruptive or flame-like. Now, continual indications of
the presence of this fire-ocean had been detected during eclipses in the
eighteenth and nineteenth centuries. Captain Stannyan, describing in a
letter to Flamsteed an occurrence of the kind witnessed by him at Berne
on May 1 (o.s.), 1706, says that the sun's "getting out of the eclipse
was preceded by a blood-red streak of light from its left limb."[176] A
precisely similar appearance was noted by both Halley and De Louville in
1715; during annular eclipses by Lord Aberdour in 1737,[177] and by
Short in 1748,[178] the tint of the ruby border being, however, subdued
to "brown" or "dusky red" by the surviving sunlight; while observations
identical in character were made at Amsterdam in 1820,[179] at Edinburgh
by Henderson in 1836, and at New York in 1838.[180]
"Flames" or "prominences," if more conspicuous, are less constant in
their presence than the glowing stratum from which they spring. The
first to describe them was a Swedish professor named Vassenius, who
observed a total eclipse at Gothenburg, May 2 (o.s.), 1733.[181] His
astonishment equalled his admiration when he perceived, just outside the
edge of the lunar disc, and suspended, as it seemed, in the coronal
atmosphere, three or four reddish spots or clouds, one of which was so
large as to be detected with the naked eye. As to their nature, he did
not even offer a speculation, further than by tacitly referring them to
the moon. The observation was repeated in 1778 by a Spanish Admiral, but
with no better success in directing efficacious attention to the
phenomenon. Don Antonio Ulloa was on board his ship the _Espagne_ in
passage from the Azores to Cape St. Vincent on the 24th of June in that
year, when a total eclipse of the sun occurred, of which he has left a
valuable description. His notices of the corona are full of interest;
but what just now concerns us is the appearance of "a red luminous
point" "near the edge of the moon," which gradually increased in size as
the moon moved away from it, and was visible during about a minute and a
quarter.[182] He was satisfied that it belonged to the sun because of
its fiery colour and growth in magnitude, and supposed that it was
occasioned by some crevice or inequality in the moon's limb, through
which the solar light penetrated.
Allusions less precise, both prior and subsequent, which it is now easy
to refer to similar objects (such as the "slender columns of smoke" seen
by Ferrer)[183] might be detailed; but the evidence already adduced
suffices to show that the prominences viewed with such amazement in 1842
were no unprecedented or even unusual phenomenon.
It was more important, however, to decide what was their nature than
whether their appearance might have been anticipated. They were
generally, and not very incorrectly, set down as solar clouds. Arago
believed them to shine by reflected light,[184] but the Abbé Peytal
rightly considered them to be self-luminous. Writing in a Montpellier
paper of July 16, 1842, he declared that we had now become assured of
the existence of a third or outer solar envelope, composed of a glowing
substance of a bright rose tint, forming mountains of prodigious
elevation, analogous in character to the clouds piled above our
horizons.[185] This first distinct recognition of a very important
feature of our great luminary was probably founded on an observation
made by Bérard at Toulon during the then recent eclipse, "of a very fine
red band, irregularly dentelated, or, as it were, crevassed here and
there,"[186] encircling a large arc of the moon's circumference. It can
hardly, however, be said to have attracted general notice until July 28,
1851. On that day a total eclipse took place, which was observed with
considerable success in various parts of Sweden and Norway by a number
of English astronomers. Mr. Hind saw, on the south limb of the moon, "a
long range of rose-coloured flames,"[187] described by Dawes as "a low
ridge of red prominences, resembling in outline the tops of a very
irregular range of hills."[188] Airy termed the portion of this "rugged
lines of projections" visible to him the _sierra_, and was struck with
its brilliant light and "nearly scarlet" colour.[189] Its true character
of a continuous solar envelope was inferred from these data by Grant,
Swan, and Littrow, and was by Father Secchi, after the great eclipse of
1860,[190] formally accepted as established.
Several prominences of remarkable forms, especially one variously
compared to a Turkish scimitar, a sickle, and a boomerang, were seen in
1851. In connection with them two highly significant circumstances were
pointed out. First, that of the approximate coincidence between their
positions and those of sun-spots previously observed.[191] Next, that
"the moon passed over them, leaving them behind, and revealing
successive portions as she advanced."[192] This latter perfectly
well-attested fact was justly considered by the Astronomer Royal and
others as affording absolute certainty of the solar dependence of these
singular objects. Nevertheless sceptics were still found. M. Faye, of
the French Academy, inclined to a lunar origin for them;[193] Feilitsch
of Greifswald published in 1852 a treatise for the express purpose of
proving all the luminous phenomena attendant on solar eclipses--corona,
prominences and "sierra"--to be purely optical appearances.[194]
Happily, however, the unanswerable arguments of the photographic camera
were soon to be made available against such hardy incredulity.
Thus, the virtual discovery of the solar appendages, both coronal and
chromospheric, may be said to have been begun in 1842, and completed in
1851. The current Herschelian theory of the solar constitution remained,
however, for the time, intact. Difficulties, indeed, were thickening
around it; but their discussion was perhaps felt to be premature, and
they were permitted to accumulate without debate, until fortified by
fresh testimony into unexpected and overwhelming preponderance.
FOOTNOTES:
[Footnote 131: Kosmos, Bd. iii., p. 409; Lalande, _Bibliographie
Astronomique_, pp. 179, 202.]
[Footnote 132: R. Wolf, _Die Sonne und ihre Flecken_, p. 9.
Marius himself, however, seems to have held the Aristotelian
terrestrial-exhalation theory of cometary origin. See his curious little
tract, _Astronomische und Astrologische Beschreibung der Cometen_,
Nürnberg, 1619.]
[Footnote 133: _Phil. Trans._, vol. xxvii., p. 274. _Umbræ_ (now called
_penumbræ_) are spaces of half-shadow which usually encircle spots.
_Faculæ_ ("little torches," so named by Scheiner) are bright streaks or
patches closely associated with spots.]
[Footnote 134: _Mém. Ac. Sc._, 1776 (pub. 1779), p. 507. D. Cassini,
however, first put forward about 1671 the hypothesis alluded to in the
text. See Delambre, _Hist. de l'Astr. Mod._, t. ii., p. 694; and
_Kosmos_, Bd. iii., p. 410.]
[Footnote 135: _Phil. Trans._, vol. lxiv., part i., pp. 7-11.]
[Footnote 136: _Rosa Ursina_, lib. iv., p. 507.]
[Footnote 137: R. Wolf, _Die Sonne und ihre Flecken_, p. 12.]
[Footnote 138: Schellen, _Die Spectralanalyse_, Bd. ii., p. 56 (3rd
ed.).]
[Footnote 139: _Phil. Trans._, vol. lxiv., p. 20.]
[Footnote 140: _Ibid._, vol. lxxxv., 1795, p. 63.]
[Footnote 141: _Phil. Trans._, vol. xci., 1801, p. 303.]
[Footnote 142: The supposed opaque or protective stratum beneath the
photosphere was named by him "planetary," from the analogy of
terrestrial clouds.]
[Footnote 143: _Ibid._, p. 305.]
[Footnote 144: _Novum Organum_, lib. ii. aph. 20.]
[Footnote 145: Brewster's _Life of Newton_, vol. ii., p. 103.]
[Footnote 146: _Beschäftigungen d. Berl. Ges. Naturforschender Freunde_,
Bd. ii., p. 233.]
[Footnote 147: _Gentleman's Magazine_, 1787, vol. ii., p. 636.]
[Footnote 148: _Results_, etc., p. 432.]
[Footnote 149: _Ibid._, p. 434.]
[Footnote 150: See _ante_, p. 31.]
[Footnote 151: _Memoir of Francis Baily, Mem. R. A. S._, vol. xv., p.
524.]
[Footnote 152: _Mem. R. A. S._, vol. x., pp. 5-6.]
[Footnote 153: _Ibid._, pp. 14-17.]
[Footnote 154: _Mem. R. A. S._, vol. xv., pp. 4-6.]
[Footnote 155: _Ibid._, p. 16.]
[Footnote 156: _Annuaire_, 1846, p. 409.]
[Footnote 157: _Ibid._, p. 317.]
[Footnote 158: _Ibid._, p. 322.]
[Footnote 159: _Phil. Trans._, vol. xl., p. 192.]
[Footnote 160: _Mem. R. A. S._, vol. x., p. 17.]
[Footnote 161: _Ann. du Bureau des Long._, 1846, p. 309.]
[Footnote 162: _De Facie in Orbe Lunæ_, xix., 10. Cf. Grant, _Astr.
Nach._, No. 1838. As to the phenomenon mentioned by Philostratus in his
_Life of Apollonius_ (viii. 23), see W. T. Lynn, _Observatory_, vol.
ix., p. 128.]
[Footnote 163: Schmidt, _Astr. Nach._, No. 1832.]
[Footnote 164: _Astronomiæ Pars Optica, Op. omnia_, t. ii., p. 317.]
[Footnote 165: _De Stellâ Novâ, Op._, t. ii., pp. 696, 697.]
[Footnote 166: _Astr. Pars Op._, p. 320.]
[Footnote 167: _Mém. de l'Ac. des Sciences_, 1706, p. 119.]
[Footnote 168: A digit = 1/12 of the solar diameter.]
[Footnote 169: _Phil. Trans._, vol. xxix., pp. 247-249.]
[Footnote 170: _Mém. de l'Ac. des Sciences_, 1715; _Histoire_, p. 49;
_Mémoires_, pp. 93-98.]
[Footnote 171: _Ibid._, 1724, p. 178.]
[Footnote 172: _Mém. de l'Ac. des Sciences_, 1715, pp. 161, 166-169.]
[Footnote 173: _Ed. Ency._, art. _Astronomy_, p. 635.]
[Footnote 174: _Trans. Am. Phil. Soc._, vol. vi., p. 274.]
[Footnote 175: _Memoir of Caroline Herschel_, p. 327.]
[Footnote 176: _Phil. Trans._, vol. xxv., p. 2240.]
[Footnote 177: _Ibid._, vol. xl., p. 182.]
[Footnote 178: _Ibid._, vol. xlv., p. 586.]
[Footnote 179: _Mem. R. A. S._, vol. i., pp. 145, 148.]
[Footnote 180: _American Journal of Science_, vol. xlii., p. 396.]
[Footnote 181: _Phil. Trans._, vol. xxxviii., p. 134. Father Secchi,
however, adverted to a distinct mention of a prominence observed in 1239
A.D. A description of a total eclipse of that date includes the remark,
"Et quoddam foramen erat ignitum in circulo solis ex parte inferiore"
(Muratori, _Rer. It. Scriptores_, t. xiv., col. 1097). The "circulus
solis" of course signifies the corona.]
[Footnote 182: _Phil. Trans._, vol. lxix., p. 114.]
[Footnote 183: _Trans. Am. Phil. Soc._, vol. vi., 1809, p. 267.]
[Footnote 184: _Annuaire_, 1846, p. 460.]
[Footnote 185: _Ibid._, p. 439, _note_.]
[Footnote 186: _Ibid._, p. 416.]
[Footnote 187: _Mem. R. A. S._, vol. xxi., p. 82.]
[Footnote 188: _Ibid._, p. 90.]
[Footnote 189: _Ibid._, pp. 7, 8.]
[Footnote 190: _Le Soleil_, t. i., p. 386.]
[Footnote 191: By Williams and Stanistreet, _Mem. R. A. S._, vol. xxi.,
pp. 54, 56. Santini had made a similar observation at Padua in 1842.
Grant, _Hist. Astr._, p. 401.]
[Footnote 192: Lassell in _Month. Not._, vol. xii., p. 53.]
[Footnote 193: _Comptes Rendus_, t. xxxiv., p. 155.]
[Footnote 194: _Optische Untersuchungen_, and _Zeitschrift für populäre
Mittheilungen_, Bd. i., 1860, p. 201.]
CHAPTER IV
_PLANETARY DISCOVERIES_
In the course of his early gropings towards a law of the planetary
distances, Kepler tried the experiment of setting a planet, invisible by
reason of its smallness, to revolve in the vast region of seemingly
desert space separating Mars from Jupiter.[195] The disproportionate
magnitude of the same interval was explained by Kant as due to the
overweening size of Jupiter. The zone in which each planet moved was,
according to the philosopher of Königsberg, to be regarded as the empty
storehouse from which its materials had been derived. A definite
relation should thus exist between the planetary masses and the
planetary intervals.[196] Lambert, on the other hand, sportively
suggested that the body or bodies (for it is noticeable that he speaks
of them in the plural) which once bridged this portentous gap in the
solar system, might, in some remote age, have been swept away by a great
comet, and forced to attend its wanderings through space.[197]
These speculations were destined before long to assume a more definite
form. Johann Daniel Titius, a professor at Wittenberg (where he died in
1796), pointed out in 1772, in a note to a translation of Bonnet's
_Contemplation de la Nature_,[198] the existence of a remarkable
symmetry in the disposition of the bodies constituting the solar system.
By a certain series of numbers, increasing in regular progression,[199]
he showed that the distances of the six known planets from the sun might
be represented with a close approach to accuracy. But with one striking
interruption. The term of the series succeeding that which corresponded
to the orbit of Mars was without a celestial representative. The orderly
flow of the sequence was thus singularly broken. The space where a
planet should--in fulfilment of the "Law"--have revolved, was, it
appeared, untenanted. Johann Elert Bode, then just about to begin his
long career as leader of astronomical thought and work at Berlin, marked
at once the anomaly, and filled the vacant interval with a hypothetical
planet. The discovery of Uranus, at a distance falling but slightly
short of perfect conformity with the law of Titius, lent weight to a
seemingly hazardous prediction, and Von Zach was actually at the pains,
in 1785, to calculate what he termed "analogical" elements[200] for this
unseen and (by any effect or influence) _unfelt_ body. The search for
it, through confessedly scarcely less chimerical than that of alchemists
for the philosopher's stone, he kept steadily in view for fifteen years,
and at length (September 21, 1800) succeeded in organising, in
combination with five other German astronomers assembled at Lilienthal,
a force of what he jocularly termed celestial police, for the express
purpose of tracking and intercepting the fugitive subject of the sun.
The zodiac was accordingly divided for purposes of scrutiny into
twenty-four zones; their apportionment to separate observers was in part
effected, and the association was rapidly getting into working order,
when news arrived that the missing planet had been found, through no
systematic plan of search, but by the diligent, though otherwise
directed labours of a distant watcher of the skies.
Giuseppe Piazzi was born at Ponte in the Valtelline, July 16, 1746. He
studied at various places and times under Tiraboschi, Beccaria,
Jacquier, and Le Sueur; and having entered the Theatine order of monks
at the age of eighteen, he taught philosophy, science, and theology in
several of the Italian cities, as well as in Malta, until 1780, when the
chair of mathematics in the University of Palermo was offered to and
accepted by him. Prince Caramanico, then viceroy of Sicily, had
scientific leanings, and was easily won over to the project of building
an observatory, a commodious foundation for which was afforded by one of
the towers of the viceregal palace. This architecturally incongruous
addition to an ancient Saracenic edifice--once the abode of Kelbite and
Zirite Emirs--was completed in February, 1791. Piazzi, meanwhile, had
devoted nearly three years to the assiduous study of his new profession,
acquiring a practical knowledge of Lalande's methods at the École
Militaire, and of Maskelyne's at the Royal Observatory; and returned to
Palermo in 1789, bringing with him, in the great five-foot circle which
he had prevailed upon Ramsden to construct, the most perfect measuring
instrument hitherto employed by an astronomer.
He had been above nine years at work on his star-catalogue, and was
still profoundly unconscious that a place amongst the Lilienthal
band[201] of astronomical detectives was being held in reserve for him,
when, on the first evening of the nineteenth century, January 1, 1801,
he noticed the position of an eighth-magnitude star in a part of the
constellation Taurus to which an error of Wollaston's had directed his
special attention. Reobserving, according to his custom, the same set of
fifty stars on four consecutive nights, it seemed to him, on the 2nd,
that the one in question had slightly shifted its position to the west;
on the 3rd he assured himself of the fact, and believed that he had
chanced upon a new kind of comet without tail or coma. The wandering
body, whatever its nature, exchanged retrograde for direct motion on
January 14,[202] and was carefully watched by Piazzi until February 11,
when a dangerous illness interrupted his observations. He had, however,
not omitted to give notice of his discovery; but so precarious were
communications in those unpeaceful times, that his letter to Oriani of
January 23 did not reach Milan until April 5, while a missive of one day
later addressed to Bode came to hand at Berlin, March 20. The delay just
afforded time for the publication, by a young philosopher of Jena named
Hegel, of a "Dissertation" showing, by the clearest light of reason,
that the number of the planets could not exceed seven, and exposing the
folly of certain devotees of induction who sought a new celestial body
merely to fill a gap in a numerical series.[203]
Unabashed by speculative scorn, Bode had scarcely read Piazzi's letter
when he concluded that it referred to the precise body in question. The
news spread rapidly, and created a profound sensation, not unmixed with
alarm lest this latest addition to the solar family should have been
found only to be again lost. For by that time Piazzi's moving star was
too near the sun to be any longer visible, and in order to rediscover it
after conjunction a tolerably accurate knowledge of its path was
indispensable. But a planetary orbit had never before been calculated
from such scanty data as Piazzi's observation afforded;[204] and the
attempts made by nearly every astronomer of note in Germany to compass
the problem were manifestly inadequate, failing even to account for the
positions in which the body had been actually seen, and _à fortiori_
serving only to mislead as to the places where, from September, 1801, it
ought once more to have become discernible. It was in this extremity
that the celebrated mathematician Gauss came to the rescue. He was then
in his twenty-fifth year, and was earning his bread by tuition at
Brunswick, with many possibilities, but no settled career before him.
The news from Palermo may be said to have converted him from an
arithmetician into an astronomer. He was already in possession of a new
and more general method of computing elliptical orbits; and the system
of "least squares," which he had devised though not published, enabled
him to extract the most probable result from a given set of
observations. Armed with these novel powers, he set to work; and the
communication in November of his elements and ephemeris for the lost
object revived the drooping hopes of the little band of eager searchers.
Their patience, however, was to be still further tried. Clouds, mist,
and sleet seemed to have conspired to cover the retreat of the fugitive;
but on the last night of the year the sky cleared unexpectedly with the
setting in of a hard frost, and there, in the north-western part of
Virgo, nearly in the position assigned by Gauss to the runaway planet, a
strange star was discerned by Von Zach[205] at Gotha, and on a
subsequent evening--the anniversary of the original discovery--by Olbers
at Bremen. The name of Ceres (as the tutelary goddess of Sicily) was, by
Piazzi's request, bestowed upon this first known of the numerous, and
probably all but innumerable family of the minor planets.
The recognition of the second followed as the immediate consequence of
the detection of the first. Olbers had made himself so familiar with the
positions of the small stars along the track of the long-missing body,
that he was at once struck (March 28, 1802) with the presence of an
intruder near the spot where he had recently identified Ceres. He at
first believed the new-comer to be a variable star usually
inconspicuous, but just then at its maximum of brightness; but within
two hours he had convinced himself that it was no _fixed_ star, but a
rapidly moving object. The aid of Gauss was again invoked, and his
prompt calculations showed that this fresh celestial acquaintance (named
"Pallas" by Olbers), revolved round the sun at nearly the same mean
distance as Ceres, and was beyond question of a strictly analogous
character.
This result was perplexing in the extreme. The symmetry and simplicity
of the planetary scheme appeared fatally compromised by the admission of
many, where room could, according to old-fashioned rules, only be found
for one. A daring hypothesis of Olbers's invention provided an exit from
the difficulty. He supposed that both Ceres and Pallas were fragments of
a primitive trans-Martian planet, blown to pieces in the remote past,
either by the action of internal forces or by the impact of a comet; and
predicted that many more such fragments would be found to circulate in
the same region. He, moreover, pointed out that these numerous orbits,
however much they might differ in other respects, must all have a common
line of intersection,[206] and that the bodies moving in them must
consequently pass, at each revolution, through two opposite points of
the heavens, one situated in the Whale, the other in the constellation
of the Virgin, where already Pallas had been found and Ceres recaptured.
The intimation that fresh discoveries might be expected in those
particular regions was singularly justified by the detection of two
bodies now known respectively as Juno and Vesta. The first was found
near the predicted spot in Cetus by Harding, Schröter's assistant at
Lilienthal, September 2, 1804; the second by Olbers himself in Virgo,
after three years of persistent scrutiny, March 29, 1807.
The theory of an exploded planet now seemed to have everything in its
favour. It required that the mean or average distances of the
newly-discovered bodies should be nearly the same, but admitted a wide
range of variety in the shapes and positions of their orbits, provided
always that they preserved common points of intersection. These
conditions were fulfilled with a striking approach to exactness. Three
of the four "asteroids" (a designation introduced by Sir. W.
Herschel[207]) conformed with very approximate precision to "Bode's law"
of distances; they all traversed, in their circuits round the sun,
nearly the same parts of Cetus and Virgo; while the eccentricities and
inclinations of their paths departed widely from the planetary
type--that of Pallas, to take an extreme instance, making with the
ecliptic an angle of nearly 35°. The minuteness of these bodies appeared
further to strengthen the imputation of a fragmentary character.
Herschel estimated the diameter of Ceres at 162, that of Pallas at 147
miles.[208] But these values are now known to be considerably too small.
A suspected variability of brightness in some of the asteroids, somewhat
hazardously explained as due to the irregularities of figure to be
expected in cosmical _potsherds_ (so to speak), was added to the
confirmatory evidence.[209] The strong point of the theory, however, lay
not in what it explained, but in what it had predicted. It had been
twice confirmed by actual exploration of the skies, and had produced, in
the recognition of Vesta, the first recorded instance of the
_premeditated_ discovery of a heavenly body.
The view not only commended itself to the facile imagination of the
unlearned, but received the sanction of the highest scientific
authority. The great Lagrange bestowed upon it his analytical
_imprimatur_, showing that the explosive forces required to produce the
supposed catastrophe came well within the bounds of possibility; since a
velocity of less than twenty times that of a cannon-ball leaving the
gun's mouth would have sufficed, according to his calculation, to launch
the asteroidal fragments on their respective paths. Indeed, he was
disposed to regard the hypothesis of disruption as more generally
available than its author had designed it to be, and proposed to
supplement with it, as explanatory of the eccentric orbits of comets,
the nebular theory of Laplace, thereby obtaining, as he said, "a
complete view of the origin of the planetary system more conformable to
Nature and mechanical laws than any yet proposed."[210]
Nevertheless the hypothesis of Olbers has not held its ground. It seemed
as if all the evidence available for its support had been produced at
once and spontaneously, while the unfavourable items were elicited
slowly, and, as it were, by cross-examination. A more extended
acquaintance with the group of bodies whose peculiarities it was framed
to explain has shown them, after all, as recalcitrant to any such
explanation. Coincidences at the first view significant and striking
have been swamped by contrary examples; and a hasty general conclusion
has, by a not uncommon destiny, at last perished under the accumulation
of particulars. Moreover, as has been remarked by Professor
Newcomb,[211] mutual perturbations would rapidly efface all traces of a
common disruptive origin, and the catastrophe, to be perceptible in its
effects, should have been comparatively recent.
A new generation of astronomers had arisen before any additions were
made to the little family of the minor planets. Piazzi died in 1826,
Harding in 1834, Olbers in 1840; all those who had prepared or
participated in the first discoveries passed away without witnessing
their resumption. In 1830, however, a certain Hencke, ex-postmaster in
the Prussian town of Driessen, set himself to watch for new planets, and
after fifteen long years his patience was rewarded. The asteroid found
by him, December 8, 1845, received the name of Astræa, and his further
prosecution of the search resulted, July 1, 1847, in the discovery of
Hebe. A few weeks later (August 13), John Russell Hind (1823-1893),
after many months' exploration from Mr. Bishop's observatory in the
Regent's Park, picked up Iris, and October 18, Flora.[212] The next on
the list was Metis, found by Mr. Graham, April 25, 1848, at Markree, in
Ireland.[213] At the close of the period to which our attention is at
present limited, the number of these small bodies known to astronomy was
thirteen; and the course of discovery has since proceeded far more
rapidly and with less interruption.
Both in itself and in its consequences the recognition of the minor
planets was of the highest importance to science. The traditional ideas
regarding the constitution of the solar system were enlarged by the
admission of a new class of bodies, strongly contrasted, yet strictly
co-ordinate with the old-established planetary order; the profusion of
resource, so conspicuous in the living kingdoms of Nature, was seen to
prevail no less in the celestial spaces; and some faint preliminary
notion was afforded of the indefinite complexity of relations underlying
the apparent simplicity of the majestic scheme to which our world
belongs. Both theoretical and practical astronomy derived profit from
the admission of these apparently insignificant strangers to the rights
of citizenship of the solar system. The disturbance of their motions by
their giant neighbours afforded a more accurate knowledge of the Jovian
mass, which Laplace had taken about 1/50 too small; the anomalous
character of their orbits presented geometers with highly stimulating
problems in the theory of perturbation; while the exigencies of the
first discovery had produced the _Theoria Motus_, and won Gauss over to
the ranks of calculating astronomy. Moreover, the sure prospect of
further detections powerfully incited to the exploration of the skies;
observers became more numerous and more zealous in view of the prizes
held out to them; star-maps were diligently constructed, and the
sidereal multitude strewn along the great zodiacal belt acquired a fresh
interest when it was perceived that its least conspicuous member might
be a planetary shred or projectile in the dignified disguise of a
distant sun. Harding's "Celestial Atlas," designed for the special
purpose of facilitating asteroidal research, was the first systematic
attempt to represent to the eye the _telescopic_ aspect of the heavens.
It was while engaged on its construction that the Lilienthal observer
successfully intercepted Juno on her passage through the Whale in 1804;
whereupon promoted to Göttingen, he there completed, in 1822, the
arduous task so opportunely entered upon a score of years previously.
Still more important were the great star-maps of the Berlin Academy,
undertaken at Bessel's suggestion, with the same object of
distinguishing errant from fixed stars, and executed, under Encke's
supervision, during the years 1830-59. They have played a noteworthy
part in the history of planetary discovery, nor of the minor kind alone.
We have now to recount an event unique in scientific history. The
discovery of Neptune has been characterised as the result of a "movement
of the age,"[214] and with some justice. It had become necessary to the
integrity of planetary theory. Until it was accomplished, the phantom of
an unexplained anomaly in the orderly movements of the solar system must
have continued to haunt astronomical consciousness. Moreover, it was
prepared by many, suggested as possible by not a few, and actually
achieved, simultaneously, independently, and completely, by two
investigators.
The position of the planet Uranus was recorded as that of a fixed star
no less than twenty times between 1690 and the epoch of its final
detection by Herschel. But these early observations, far from affording
the expected facilities for the calculation of its orbit, proved a
source of grievous perplexity. The utmost ingenuity of geometers failed
to combine them satisfactorily with the later Uranian places, and it
became evident, either that they were widely erroneous, or that the
revolving body was wandering from its ancient track. The simplest course
was to reject them altogether, and this was done in the new Tables
published in 1821 by Alexis Bouvard, the indefatigable computating
partner of Laplace. But the trouble was not thus to be got rid of. After
a few years fresh irregularities began to appear, and continued to
increase until absolutely "intolerable." It may be stated as
illustrative of the perfection to which astronomy had been brought, that
divergencies regarded as menacing the very foundation of its theories
never entered the range of unaided vision. In other words, if the
theoretical and the real Uranus had been placed side by side in the sky,
they would have seemed, to the sharpest eye, to form a single body.[215]
The idea that these enigmatical disturbances were due to the attraction
of an unknown exterior body was a tolerably obvious one; and we
accordingly find it suggested in many different quarters. Bouvard
himself was perhaps the first to conceive it. He kept the possibility
continually in view, and bequeathed to his nephew's diligence the
inquiry into its reality when he felt that his own span was drawing to a
close; but before any progress had been made with it, he had already
(June 7, 1843) "ceased to breathe and to calculate." The Rev. T. J.
Hussey actually entertained in 1834 the notion, but found his powers
inadequate to the task, of assigning an approximate place to the
disturbing body; and Bessel, in 1840, laid his plans for an assault in
form upon the Uranian difficulty, the triumphant exit from which fatal
illness frustrated his hopes of effecting or even witnessing.
The problem was practically untouched when, in 1841, an undergraduate of
St. John's College, Cambridge, formed the resolution of grappling with
it. The projected task was an arduous one. There were no guiding
precedents for its conduct. Analytical obstacles had to be encountered
so formidable as to appear invincible even to such a mathematician as
Airy. John Couch Adams, however, had no sooner taken his degree, which
he did as senior wrangler in January, 1843, than he set resolutely to
work, and on October 21, 1845, was able to communicate to the Astronomer
Royal numerical estimates of the elements and mass of the unknown
planet, together with an indication of its actual place in the heavens.
These results, it has been well said,[216] gave "the final and
inexorable proof" of the validity of Newton's Law. The date October 21,
1845, "may therefore be regarded as marking a distinct epoch in the
history of gravitational astronomy."
Sir George Biddell Airy had begun in 1835 his long and energetic
administration of the Royal Observatory, and was already in possession
of data vitally important to the momentous inquiry then on foot. At his
suggestion, and under his superintendence, the reduction of all the
planetary observations made at Greenwich from 1750 onwards had been
undertaken in 1833. The results, published in 1846, constituted a
permanent and universal stock of materials for the correction of
planetary theory. But in the meantime, investigators, both native and
foreign, were freely supplied with the "places and errors," which,
clearly exhibiting the discrepancies between observation and
calculation--between what _was_ and what was _expected_--formed the very
groundwork of future improvements.
Mr. Adams had no reason to complain of official discourtesy. His labours
received due and indispensable aid; but their purpose was regarded as
chimerical. "I have always," Sir George Airy wrote,[217] "considered the
correctness of a distant mathematical result to be a subject rather of
moral than of mathematical evidence." And that actually before him
seemed, from its very novelty, to incur a suspicion of unlikelihood. No
problem in planetary disturbance had heretofore been attacked, so to
speak, from the rear. The inverse method was untried, and might well be
deemed impracticable. For the difficulty of determining the
perturbations produced by a given planet is small compared with the
difficulty of finding a planet by its resulting perturbations. Laplace
might have quailed before it; yet it was now grappled with as a first
essay in celestial dynamics. Moreover, Adams unaccountably neglected to
answer until too late a question regarded by Airy in the light of an
_experimentum crucis_ as to the soundness of the new theory. Nor did he
himself take any steps to obtain a publicity which he was more anxious
to merit than to secure. The investigation consequently remained buried
in obscurity. It is now known that had a search been instituted in the
autumn of 1845 for the remote body whose existence had been so
marvellously foretold, it would have been found within _three and a half
lunar diameters_ (1° 49') of the spot assigned to it by Adams.
A competitor, however, equally daring and more fortunate--_audax fortunâ
adjutus_, as Gauss said of him--was even then entering the field. Urbain
Jean Joseph Leverrier, the son of a small Government _employé_ in
Normandy, was born at Saint-Lô, March 11, 1811. He studied with
brilliant success at the École Polytechnique, accepted the post of
astronomical teacher there in 1837, and, "docile to circumstance,"
immediately concentrated the whole of his vast, though as yet
undeveloped powers upon the formidable problems, of celestial mechanics.
He lost no time in proving to the mathematical world that the race of
giants was not extinct. Two papers on the stability of the solar system,
presented to the Academy of Sciences, September 16 and October 14, 1839,
showed him to be the worthy successor of Lagrange and Laplace, and
encouraged hopes destined to be abundantly realised. His attention was
directed by Arago to the Uranian difficulty in 1845, when he cheerfully
put aside certain intricate cometary researches upon which he happened
to be engaged, in order to obey with dutiful promptitude the summons of
the astronomical chief of France. In his first memoir on the subject
(communicated to the Academy, November 10, 1845), he proved the
inadequacy of all known causes of disturbance to account for the
vagaries of Uranus; in a second (June 1, 1848), he demonstrated that
only an exterior body, occupying at a certain date a determinate
position in the zodiac, could produce the observed effects; in a third
(August 31, 1846), he assigned the orbit of the disturbing body, and
announced its visibility as an object with a sensible disc about as
bright as a star of the eighth magnitude.
The question was now visibly approaching an issue. On September 10, Sir
John Herschel declared to the British Association respecting the
hypothetical new planet: "We see it as Columbus saw America from the
coast of Spain. Its movements have been felt, trembling along the
far-reaching line of our analysis with a certainty hardly inferior to
that of ocular demonstration." Less than a fortnight later, September
23, Professor Galle, of the Berlin Observatory, received a letter from
Leverrier requesting his aid in the telescopic part of the inquiry
already analytically completed. He directed his refractor to the heavens
that same night, and perceived, within less than a degree of the spot
indicated, an object with a measurable disc nearly three seconds in
diameter. Its absence from Bremiker's recently-completed map of that
region of the sky showed it to be no star, and its movement in the
predicted direction confirmed without delay the strong persuasion of its
planetary nature.[218]
In this remarkable manner the existence of the remote member of our
system known as "Neptune" was ascertained. But the discovery, which
faithfully reflected the duplicate character of the investigation which
led to it, had been already secured at Cambridge before it was announced
from Berlin. Sir George Airy's incredulity vanished in the face of the
striking coincidence between the position assigned by Leverrier to the
unknown planet in June, and that laid down by Adams in the previous
October; and on the 9th of July he wrote to Professor Challis, director
of the Cambridge Observatory, recommending a search with the great
Northumberland equatoreal. Had a good star-map been at hand, the process
would have been a simple one; but of Bremiker's "Hora XXI." no news had
yet reached England, and there was no other sufficiently comprehensive
to be available for an inquiry which, in the absence of such aid,
promised to be both long and laborious. As the event proved, it might
have been neither. "After four days of observing," Challis wrote,
October 12, 1846, to Airy, "the planet was in my grasp if only I had
examined or mapped the observations."[219] Had he done so, the first
honours in the discovery, both theoretical and optical, would have
fallen to the University of Cambridge. But Professor Challis had other
astronomical avocations to attend to, and, moreover, his faith in the
precision of the indications furnished to him was, by his own
confession, a very feeble one. For both reasons he postponed to a later
stage of the proceedings the discussion and comparison of the data
nightly furnished to him by his telescope, and thus allowed to lie, as
it were, latent in his observations the momentous result which his
diligence had insured, but which his delay suffered to be
anticipated.[220]
Nevertheless, it should not be forgotten that the Berlin astronomer had
two circumstances in his favour apart from which his swift success could
hardly have been achieved. The first was the possession of a good
star-map; the second was the clear and confident nature of Leverrier's
instructions. "Look where I tell you," he seemed authoritatively to say,
"and you will see an object such as I describe."[221] And in fact, not
only Galle on the 23rd of September, but also Challis on the 29th,
immediately after reading the French geometer's lucid and impressive
treatise, picked out from among the stellar points strewing the zodiac,
a small planetary disc, which eventually proved to be that of the
precise body he had been in search of during two months.
The controversy that ensued had its ignominious side; but it was entered
into by neither of the parties principally concerned. Adams bore the
disappointment, which the dilatory proceedings at Greenwich and
Cambridge had inflicted upon him, with quiet heroism. His silence on the
subject of what another man would have called his wrongs remained
unbroken to the end of his life;[222] and he took every opportunity of
testifying his admiration for the genius of Leverrier.
Personal questions, however, vanish in the magnitude of the event they
relate to. By it the last lingering doubts as to the absolute exactness
of the Newtonian Law were dissipated. Recondite analytical methods
received a confirmation brilliant and intelligible even to the minds of
the vulgar, and emerged from the patient solitude of the study to enjoy
an hour of clamorous triumph. For ever invisible to the unaided eye of
man, a sister-globe to our earth was shown to circulate, in perpetual
frozen exile, at thirty times its distance from the sun. Nay, the
possibility was made apparent that the limits of our system were not
even thus reached, but that yet profounder abysses of space might
shelter obedient, though little favoured, members of the solar family,
by future astronomers to be recognised through the sympathetic
thrillings of Neptune, even as Neptune himself was recognised through
the tell-tale deviations of Uranus.
It is curious to find that the fruit of Adams's and Leverrier's
laborious investigations had been accidentally all but snatched half a
century before it was ripe to be gathered. On the 8th, and again on the
10th of May, 1795, Lalande noted the position of Neptune as that of a
fixed star, but perceiving that the two observations did not agree, he
suppressed the first as erroneous, and pursued the inquiry no further.
An immortality which he would have been the last to despise hung in the
balance; the feather-weight of his carelessness, however, kicked the
beam, and the discovery was reserved to be more hardly won by later
comers.
Bode's Law did good service in the quest for a trans-Uranian planet by
affording ground for a probable assumption as to its distance. A
starting-point for approximation was provided by it; but it was soon
found to be considerably at fault. Even Uranus is about 36 millions of
miles nearer to the sun than the order of progression requires; and
Neptune's vast distance of 2,800 million should be increased by no less
than 800 million miles, and its period of 165 lengthened out to 225
years,[223] in order to bring it into conformity with the curious and
unexplained rule which planetary discoveries have alternately tended to
confirm and to invalidate.
Within seventeen days of its identification with the Berlin achromatic,
Neptune was found to be attended by a satellite. This discovery was the
first notable performance of the celebrated two-foot reflector[224]
erected by Mr. Lassell at his suggestively named residence of Starfield,
near Liverpool. William Lassell was a brewer by profession, but by
inclination an astronomer. Born at Bolton in Lancashire, June 18, 1799,
he closed a life of eminent usefulness to science, October 5, 1818, thus
spanning with his well-spent years four-fifths of the momentous period
which we have undertaken to traverse. At the age of twenty-one, being
without the means to purchase, he undertook to construct telescopes, and
naturally turned his attention to the reflecting sort, as favouring
amateur efforts by the comparative simplicity of its structure. His
native ingenuity was remarkable, and was developed by the hourly
exigencies of his successive enterprises. Their uniform success
encouraged him to enlarge his aims, and in 1844 he visited Birr Castle
for the purpose of inspecting the machine used in polishing the giant
speculum of Parsonstown. In the construction of his new instrument,
however, he eventually discarded the model there obtained, and worked on
a method of his own, assisted by the supreme mechanical skill of James
Nasmyth. The result was a Newtonian of exquisite definition, with an
aperture of two, and a focal length of twenty feet, provided by a novel
artifice with the equatoreal mounting, previously regarded as available
only for refractors.
This beautiful instrument afforded to its maker, October 10, 1846, a
cursory view of a Neptunian attendant. But the planet was then
approaching the sun, and it was not until the following July that the
observation could be verified, which it was completely, first by Lassell
himself, and somewhat later by Otto Stuve and Bond of Cambridge (U.S.).
When it is considered that this remote object shines by reflecting
sunlight reduced by distance to 1/900th of the intensity with which it
illuminates our moon, the fact of its visibility, even in the most
perfect telescopes, is a somewhat surprising one. It can only, indeed,
be accounted for by attributing to it dimensions very considerable for a
body of the secondary order. It shares with the moons of Uranus the
peculiarity of retrograde motion; that is to say, its revolutions,
running counter to the grand current of movement in the solar system,
are performed from east to west, in a plane inclined at an angle of 35°
to that of the ecliptic. Their swiftness serves to measure the mass of
the globe round which they are performed. For while our moon takes
twenty-seven days and nearly eight hours to complete its circuit of the
earth, the satellite of Neptune, at a distance not greatly inferior,
sweeps round its primary in five days and twenty-one hours, showing
(according to a very simple principle of computation) that it is urged
by a force seventeen times greater than the terrestrial pull upon the
lunar orb. Combining this result with those of Professor Barnard's[225]
and Dr. See's[226] recent measurements of the small telescopic disc of
this farthest known planet, it is found that while in _mass_ Neptune
equals seventeen, in _bulk_ it is equivalent to forty-nine earths. This
is as much as to say that it is composed of relatively very light
materials, or more probably of materials distended by internal heat, as
yet unwasted by radiation into space, to about five times the volume
they would occupy in the interior of our globe. The fact, at any rate,
is fairly well ascertained, that the average density of Neptune is about
twice that of water.
We must now turn from this late-recognised member of our system to
bestow some brief attention upon the still fruitful field of discovery
offered by one of the immemorial five. The family of Saturn, unlike that
of its brilliant neighbour, has been gradually introduced to the notice
of astronomers. Titan, the sixth Saturnian moon in order of distance,
led the way, being detected by Huygens, March 25, 1655; Cassini made the
acquaintance of four more between 1671 and 1684; while Mimas and
Enceladus, the two innermost, were caught by Herschel in 1789, as they
threaded their lucid way along the edge of the almost vanished ring. In
the distances of these seven revolving bodies from their primary, an
order of progression analogous to that pointed out by Titius in the
planetary intervals was found to prevail; but with one conspicuous
interruption, similar to that which had first suggested the search for
new members of the solar system. Between Titan and Japetus--the sixth
and seventh reckoning outwards--there was obviously room for another
satellite. It was discovered on both sides of the Atlantic
simultaneously, on the 19th of September, 1848. Mr. W. C. Bond,
employing the splendid 15-inch refractor of the Harvard Observatory,
noticed, September 16, a minute star situated in the plane of Saturn's
rings. The same object was discerned by Mr. Lassell on the 18th. On the
following evening, both observers perceived that the problematical speck
of light kept up with, instead of being left behind by the planet as it
moved, and hence inferred its true character.[227] Hyperion, the seventh
by distance and eighth by recognition of Saturn's attendant train, is of
so insignificant a size when compared with some of its fellow-moons
(Titan is but little inferior to the planet Mars), as to have suggested
to Sir John Herschel[228] the idea that it might be only one of several
bodies revolving very close together--in fact, an _asteroidal
satellite_; but the conjecture has, so far, not been verified.
The coincidence of its duplicate discovery was singularly paralleled two
years later. Galileo's amazement when his "optic glass" revealed to him
the "triple" form of Saturn--_planeta tergeminus_--has proved to be,
like the laughter of the gods, "inextinguishable." It must revive in
every one who contemplates anew the unique arrangements of that world
apart known to us as the Saturnian system. The resolution of the
so-called _ansæ_, or "handles," into one encircling ring by Huygens in
1655, the discovery by Cassini in 1675 of the division of that ring into
two concentric ones, together with Laplace's investigation of the
conditions of stability of such a formation, constituted, with some
minor observations, the sum of the knowledge obtained, up to the middle
of the last century, on the subject of this remarkable formation. The
first place in the discovery now about to be related belongs to an
American astronomer.
William Cranch Bond, born in 1789 at Portland, in the State of Maine,
was a watchmaker, whom the solar eclipse of 1806 attracted to study the
wonders of the heavens. When, in 1815, the erection of an observatory in
connection with Harvard College, Cambridge, was first contemplated, he
undertook a mission to England for the purpose of studying the working
of similar institutions there, and on his return erected a private
observatory at Dorchester, where he worked diligently for many years.
Then at last, in 1843, the long-postponed design of the Harvard
authorities was resumed, and on the completion of the new establishment,
Bond, who had been from 1838 officially connected with the College and
had carried on his scientific labours within its precincts, was offered
and accepted the post of its director. Placed in 1847 in possession of
one of the finest instruments in the world--a masterpiece of Merz and
Mahler--he headed the now long list of distinguished Transatlantic
observers. Like the elder Struve, he left an heir to his office and to
his eminence, but George Bond unfortunately died in 1865, at the early
age of thirty-nine, having survived his father but six years.
On the night of November 15, 1850--the air, remarkably enough, being so
hazy that only the brightest stars could be perceived with the naked
eye--William Bond discerned a dusky ring, extending about halfway
between the inner brighter one and the globe of Saturn. A fortnight
later, but before the observation had been announced in England, the
same appearance was seen by the Rev. W. R. Dawes with the comparatively
small refractor of his observatory at Wateringbury, and on December 3
was described by Mr. Lassell (then on a visit to him) as "something like
a crape veil covering a part of the sky within the inner ring."[229]
Next morning the _Times_ containing the report of Bond's discovery
reached Wateringbury. The most surprising circumstance in the matter was
that the novel appendage had remained so long unrecognised. As the rings
opened out to their full extent, it became obvious with very moderate
optical assistance; yet some of the most acute observers who have ever
lived, using instruments of vast power, had heretofore failed to detect
its presence. It soon appeared, however, that Galle of Berlin[230] had
noticed, June 10, 1838, a veil-like extension of the lucid ring across
half the dark space separating it from the planet; but the observation,
although communicated at the time to the Berlin Academy of Sciences, had
remained barren. Traces of the dark ring, moreover, were found in
drawings executed by Campani in 1664[231] and by Hooke in 1666;[232]
while Picard (June 15, 1673),[233] Hadley (spring of 1720),[234] and
Herschel,[235] had all undoubtedly seen it under the aspect of a dark
bar or belt crossing the Saturnian globe. It was, then, of no recent
origin; but there seemed reason to think that it had lately gained
considerably in brightness. The full meaning of this suspected change it
was reserved for later investigations to develop.
What we may, in a certain sense, call the closing result of the race for
discovery, in which several observers seemed at that time to be engaged,
was the establishment, on a satisfactory footing, of our acquaintance
with the dependent system of Uranus. Sir William Herschel, whose
researches formed, in so many distinct lines of astronomical inquiry,
the starting-points of future knowledge, detected, January 11,
1787,[236] two Uranian moons, since called Oberon and Titania, and
ascertained the curious circumstance of their motion in a plane almost
at right angles to the ecliptic, in a direction contrary to that of all
previously known denizens (other than cometary) of the solar kingdom. He
believed that he caught occasional glimpses of four more, but never
succeeded in assuring himself of their substantial existence. Even the
two first remained unseen save by himself until 1828, when his son
re-observed them with a 20-foot reflector, similar to that with which
they had been originally discovered. Thenceforward they were kept fairly
within view, but their four questionable companions, in spite of some
false alarms of detection, remained in the dubious condition in which
Herschel had left them. At last, on October 24, 1851,[237] after some
years of fruitless watching, Lassell espied "Ariel" and "Umbriel," two
Uranian attendants, interior to Oberon and Titania, and of about half
their brightness; so that their disclosure is still reckoned amongst the
very highest proofs of instrumental power and perfection. In all
probability they were then for the first time seen; for although
Professor Holden[238] made out a plausible case in favour of the fitful
visibility to Herschel of each of them in turn, Lassell's argument[239]
that the glare of the planet in Herschel's great specula must have
rendered almost impossible the perception of objects so minute and so
close to its disc, appears tolerably decisive to the contrary. Uranus is
thus attended by four moons, and, so far as present knowledge extends,
by no more. Among the most important of the "negative results"[240]
secured by Lassell's observations at Malta during the years 1852-53 and
1861-65, were the convincing evidence afforded by them that, without
great increase of optical power, no further Neptunian or Uranian
satellites can be perceived, and the consequent relegation of Herschel's
baffling quartette, notwithstanding the unquestioned place long assigned
to them in astronomical text-books, to the Nirvana of non-existence.
FOOTNOTES:
[Footnote 195: _Op._, t. i., p. 107. He interposed, but tentatively
only, another similar body between Mercury and Venus.]
[Footnote 196: _Allgemeine Naturgeschichte_ (ed. 1798), pp. 118, 119.]
[Footnote 197: _Cosmologische Briefe_, No. 1 (quoted by Von Zach,
_Monat. Corr._, vol. iii., p. 592).]
[Footnote 198: Second ed., p. 7. See Bode, _Von dem neuen
Hauptplaneten_, p. 43, _note_.]
[Footnote 199: The representative numbers are obtained by adding 1 to
the following series (irregular, it will be observed, in its first
member, which should be 1/2 instead of 0); 0, 3, 6, 12, 24, 48, etc. The
formula is a purely empirical one, and is, moreover, completely at fault
as regards the distance of Neptune.]
[Footnote 200: _Monat. Corr._, vol. iii., p. 596.]
[Footnote 201: Wolf, _Geschichte der Astronomie_, p. 648.]
[Footnote 202: Such reversals of direction in the apparent movements of
the planets are a consequence of the earth's revolution in its orbit.]
[Footnote 203: _Dissertatio Philosophica de Orbitis Planetarum_, 1801.
See Wolf, _Gesch. d. Astr._, p. 685.]
[Footnote 204: Observations on Uranus, as a supposed fixed star, went
back to 1690.]
[Footnote 205: He had caught a glimpse of it on December 7, but was
prevented by bad weather from verifying his suspicion. _Monat. Corr._,
vol. v., p. 171.]
[Footnote 206: Planetary fragments, hurled _in any direction_, and _with
any velocity_ short of that which would for ever release them from the
solar sway, would continue to describe elliptic orbits round the sun,
all passing through the scene of the explosion, and thus possessing a
common line of intersection.]
[Footnote 207: _Phil. Trans._, vol. xcii., part ii., p. 228.]
[Footnote 208: _Ibid._, p. 218. In a letter to Von Zach of June 24,
1802, he speaks of Pallas as "almost incredibly small," and makes it
only seventy English miles in diameter. _Monat. Corr._, vol. vi., pp.
89, 90.]
[Footnote 209: Olbers, _Monat. Corr._, vol. vi., p. 88.]
[Footnote 210: _Conn. d. Tems_ for 1814, p. 218.]
[Footnote 211: _Popular Astronomy_, p. 327.]
[Footnote 212: _Month. Not._, vol. vii., p. 299; vol. viii., p. 1.]
[Footnote 213: _Ibid._, p. 146.]
[Footnote 214: Airy, _Mem. R. A. S._, vol. xvi., p. 386.]
[Footnote 215: See Newcomb's _Pop. Astr._, p. 359. The error of Uranus
amounted, in 1844, to 2'; but even the tailor of Breslau, whose
extraordinary powers of vision Humboldt commemorates (_Kosmos_, Bd. ii.,
p. 112), could only see Jupiter's first satellite at its greatest
elongation, 2' 15". He might, however, possibly have distinguished two
objects of _equal_ lustre at a lesser interval.]
[Footnote 216: J. W. L. Glaisher, _Observatory_, vol. xv., p. 177.]
[Footnote 217: _Mem. R. A. S._, vol. xvi., p. 399.]
[Footnote 218: For an account of D'Arrest's share in the detection see
_Copernicus_, vol. ii., pp. 63, 96.]
[Footnote 219: _Mem. R. A. S._, vol. xvi., p. 412.]
[Footnote 220: He had recorded the places of 3,150 stars (three of which
were different positions of the planet), and was preparing to map them,
when, October 1, news of the discovery arrived from Berlin. Prof.
Challis's _Report_, quoted in Obituary Notice, _Month. Not._, Feb.,
1883, p. 170.]
[Footnote 221: See Airy in _Mem. R. A. S._, vol. xvi., p. 411.]
[Footnote 222: He died January 21, 1892, in his 71st year.]
[Footnote 223: Ledger, _The Sun, its Planets and their Satellites_, p.
414.]
[Footnote 224: Presented by the Misses Lassell, after their father's
death, to the Greenwich Observatory.]
[Footnote 225: _Astr. Jour._, No. 508.]
[Footnote 226: _Report of U.S. Naval Observatory for 1900_, p. 15.]
[Footnote 227: Grant, _Hist. of Astr._, p. 271.]
[Footnote 228: _Month. Not._, vol. ix., p. 91.]
[Footnote 229: _Month. Not._, vol. xi., p. 21.]
[Footnote 230: _Astr. Nach._, No. 756 (May 2, 1851).]
[Footnote 231: _Phil. Trans._, vol. i., p. 246. See H. T. Vivian, _Engl.
Mech._, April 20, 1894.]
[Footnote 232: Secchi, _Month. Not._, vol. xiii., p. 248.]
[Footnote 233: Hind, _ibid._, vol. xv., p. 32.]
[Footnote 234: Lynn, _Observatory_, Oct. 1, 1883; Hadley, _Phil.
Trans._, vol. xxxii., p. 385.]
[Footnote 235: Proctor, _Saturn and its System_, p. 64.]
[Footnote 236: _Phil. Trans._, vol. lxxvii., p. 125.]
[Footnote 237: _Month. Not._, vol. xi., p. 248.]
[Footnote 238: _Ibid._, vol. xxxv., pp. 16-22.]
[Footnote 239: _Ibid._, p. 26.]
[Footnote 240: _Ibid._, vol. xli., p. 190.]
CHAPTER V
_COMETS_
Newton showed that the bodies known as "comets," or _hirsute_ stars,
obey the law of gravitation; but it was by no means certain that the
individual of the species observed by him in 1680 formed a permanent
member of the solar system. The velocity, in fact, of its rush round the
sun was quite possibly sufficient to carry it off for ever into the
depths of space, there to wander, a celestial casual, from star to star.
With another comet, however, which appeared two years later, the case
was different. Edmund Halley, who afterwards succeeded Flamsteed as
Astronomer Royal, calculated the elements of its orbit on Newton's
principles, and found them to resemble so closely those similarly
arrived at for comets observed by Peter Apian in 1531, and by Kepler in
1607, as almost to compel the inference that all three were apparitions
of a single body. This implied its revolution in a period of about
seventy-six years, and Halley accordingly fixed its return for 1758-9.
So fully alive was he to the importance of the announcement that he
appealed to a "candid posterity," in the event of its verification, to
acknowledge that the discovery was due to an Englishman. The prediction
was one of the test-questions put by Science to Nature, on the replies
to which largely depend both the development of knowledge and the
conviction of its reality. In the present instance, the answer afforded
may be said to have laid the foundation of this branch of astronomy.
Halley's comet punctually reappeared on Christmas Day, 1758, and
effected its perihelion passage on the 12th of March following, thus
proving beyond dispute that some at least of these erratic bodies are
domesticated within our system, and strictly conform, if not to its
unwritten customs (so to speak), at any rate to its fundamental laws.
Their movements, in short, were demonstrated by the most unanswerable of
all arguments--that of verified calculation--to be _calculable_, and
their investigation was erected into a legitimate department of
astronomical science.
This notable advance was the chief _result_ obtained in the field of
inquiry just now under consideration during the eighteenth century. But
before it closed, its cultivation had received a powerful stimulus
through the invention of an improved _method_. The name of Olbers has
already been brought prominently before our readers in connection with
asteroidal discoveries; these, however, were but chance excursions from
the path of cometary research which he steadily pursued through life. An
early predilection for the heavens was fixed in this particular
direction by one of the happy inspirations of genius. As he was
watching, one night in the year 1779, by the sick-bed of a
fellow-student in medicine at Göttingen, an important simplification in
the mode of computing the paths of comets occurred to him. Although not
made public until 1797, "Olbers's method" was then universally adopted,
and is still regarded as the most expeditious and convenient in cases
where absolute rigour is not required. By its introduction, not only
many a toilsome and thankless hour was spared, but workers were
multiplied, and encouraged in the prosecution of labours more useful
than attractive.
The career of Heinrich Olbers is a brilliant example of what may be done
by an amateur in astronomy. He at no time did regular work in an
observatory; he was never the possessor of a transit or any other fixed
instrument; moreover, all the best years of his life were absorbed in
the assiduous exercise of a toilsome profession. Born in 1758 at the
village of Arbergen, where his father was pastor, he settled in 1781 as
a physician in the neighbouring town of Bremen, and continued in active
practice there for over forty years. It was thus only the hours which
his robust constitution enabled him to spare from sleep that were
available for his intellectual pleasures. Yet his recreation was, as Von
Zach remarked,[241] no less prolific of useful results than the severest
work of other men. The upper part of his house in the Sandgasse was
fitted up with such instruments and appliances as restrictions of space
permitted, and there, night after night during half a century and
upwards, he discovered, calculated, or observed the cometary visitants
of northern skies. Almost as effective in promoting the interests of
science as the valuable work actually done by him, was the influence of
his genial personality. He engaged confidence by his ready and
discerning sympathy; he inspired affection by his benevolent
disinterestedness; he quickened thought and awakened zeal by the
suggestions of a lively and inventive spirit, animated with the warmest
enthusiasm for the advancement of knowledge. Nearly every astronomer in
Germany enjoyed the benefits of a frequently active correspondence with
him, and his communications to the scientific periodicals of the time
were numerous and striking. The motive power of his mind was thus widely
felt and continually in action. Nor did it wholly cease to be exerted
even when the advance of age and the progress of infirmity rendered him
incapable of active occupation. He was, in fact, _alive_ even to the
last day of his long life of eighty-one years; and his death, which
occurred March 2, 1840, left vacant a position which a rare combination
of moral and intellectual qualities had conspired to render unique.
Amongst the many younger men who were attracted and stimulated by
intercourse with him was Johann Franz Encke. But while Olbers became a
mathematician because he was an astronomer, Encke became an astronomer
because he was a mathematician. A born geometer, he was naturally sent
to Göttingen and placed under the tuition of Gauss. But geometers are
men; and the contagion of patriotic fervour which swept over Germany
after the battle of Leipsic did not spare Gauss's promising pupil. He
took up arms in the Hanseatic Legion, and marched and fought until the
oppressor of his country was safely ensconced behind the ocean-walls of
St. Helena. In the course of his campaigning he met Lindenau, the
militant director of the Seeberg Observatory, and by his influence was
appointed his assistant, and eventually, in 1822, became his successor.
Thence he was promoted in 1825 to Berlin, where he superintended the
building of the new observatory, so actively promoted by Humboldt, and
remained at its head until within some eighteen months of his death in
August, 1865.
On the 26th of November, 1818, Pons of Marseilles discovered a comet,
whose inconspicuous appearance gave little promise of its becoming one
of the most interesting objects in our system. Encke at once took the
calculation of its elements in hand, and brought out the unexpected
result that it revolved round the sun in a period of about 3-1/3
years.[242] He, moreover, detected its identity with comets seen by
Méchain in 1786, by Caroline Herschel in 1795, by Pons, Huth, and
Bouvard in 1805, and after six laborious weeks of research into the
disturbances experienced by it from the planets during the entire
interval since its first ascertained appearance, he fixed May 24, 1822,
as the date of its next return to perihelion. Although on that occasion,
owing to the position of the earth, invisible in the northern
hemisphere, Sir Thomas Brisbane's observatory at Paramatta was
fortunately ready equipped for its recapture, which Rümker effected
quite close to the spot indicated by Encke's ephemeris.
The importance of this event can be better understood when it is
remembered that it was only the second instance of the recognised return
of a comet (that of Halley's, sixty-three years previously, having, as
already stated, been the first); and that it, moreover, established the
existence of a new class of celestial objects, somewhat loosely
distinguished as "comets of short period." These bodies (of which about
thirty have been found to circulate within the orbit of Saturn) are
remarkable as showing certain planetary affinities in the manners of
their motions not at all perceptible in the wider travelling members of
their order. They revolve, without exception, in the same direction as
the planets--from west to east; they exhibit a marked tendency to
conform to the zodiacal track which limits planetary excursions north
and south; and their paths round the sun, although much more eccentric
than the approximately circular planetary orbits, are far less so than
the extravagantly long ellipses in which comets comparatively untrained
(as it were) in the habits of the solar system ordinarily perform their
revolutions.
No _great_ comet is of the "planetary" kind. These are, indeed, only by
exception visible to the naked eye; they possess extremely feeble
tail-producing powers, and give small signs of central condensation.
Thin wisps of cosmical cloud, they flit across the telescopic field of
view without sensibly obscuring the smallest star. Their appearance, in
short, suggests--what some notable facts in their history will presently
be shown to confirm--that they are bodies already effete, and verging
towards dissolution. If it be asked what possible connection can be
shown to exist between the shortness of period by which they are
essentially characterised, and what we may call their _superannuated_
condition, we are not altogether at a loss for an answer. Kepler's
remark,[243] that comets are consumed by their own emissions, has
undoubtedly a measure of truth in it. The substance ejected into the
tail must, in overwhelmingly large proportion, be for ever lost to the
central mass from which it issues. True, it is of a nature inconceivably
tenuous; but unrepaired waste, however small in amount, cannot be
persisted in with impunity. The incitement to such self-spoliation
proceeds from the sun; it accordingly progresses more rapidly the more
numerous are the returns to the solar vicinity. Comets of short period
may thus reasonably be expected to _wear out_ quickly.
They are, moreover, subject to many adventures and vicissitudes. Their
aphelia--or the farthest points of their orbits from the sun--are
usually, if not invariably, situated so near to the path either of
Jupiter or of Saturn, as to permit these giant planets to act as
secondary rulers of their destinies. By their influence they were, in
all likelihood, originally fixed in their present tracks; and by their
influence, exerted in an opposite sense, they may, in some cases, be
eventually ejected from them. Careers so varied, as can easily be
imagined, are apt to prove instructive, and astronomers have not been
backward in extracting from them the lessons they are fitted to convey.
Encke's comet, above all, has served as an index to much curious
information, and it may be hoped that its function in that respect is by
no means at an end. The great extent of the solar system traversed by
its eccentric path makes it peculiarly useful for the determination of
the planetary masses. At perihelion it penetrates within the orbit of
Mercury; it considerably transcends at aphelion the farthest excursion
of Pallas. Its vicinity to the former planet in August, 1835, offered
the first convenient opportunity of placing that body in the
astronomical balance. Its weight or mass had previously been assumed,
not ascertained; and the comparatively slight deviation from its regular
course impressed upon the comet by its attractive power showed that it
had been assumed nearly twice too great.[244] That fundamental datum of
planetary astronomy--the mass of Jupiter--was corrected by similar
means; and it was reassuring to find the correction in satisfactory
accord with that already introduced from observations of the asteroidal
movements.
The fact that comets contract in approaching the sun had been noticed by
Hevelius; Pingré admitted it with hesitating perplexity;[245] the
example of Encke's comet rendered it conspicuous and undeniable. On the
28th of October, 1828, the diameter of the nebulous matter composing
this body was estimated at 312,000 miles. It was then about one and a
half times further from the sun than the earth is at the time of the
equinox. On the 24th of December following, its distance being reduced
by nearly two-thirds, it was found to be only 14,000 miles across.[246]
That is to say, it had shrunk during those two months of approach to
1/11000th part of its original volume! Yet it had still seventeen days'
journey to make before reaching perihelion. The same curious
circumstance was even more markedly apparent at its return in 1838. Its
bulk, or the actual space occupied by it, appeared to be reduced, as it
drew near the hearth of our system, in the enormous proportion of
800,000 to 1. A corresponding expansion accompanied on each occasion its
retirement from the sphere of observation. Similar changes of volume,
though rarely to the same astounding extent, have been perceived in
other comets. They still remain unexplained; but it can scarcely be
doubted that they are due to the action of the same energetic internal
forces which reveal themselves in so many splendid and surprising
cometary phenomena.
Another question of singular interest was raised by Encke's acute
inquiries into the movements and disturbances of the first known "comet
of short period." He found from the first that its revolutions were
subject to some influence besides that of gravity. After every possible
allowance had been made for the pulls, now backward, now forward,
exerted upon it by the several planets, there was still a surplus of
acceleration left unaccounted for. Each return to perihelion took place
about two and a half hours sooner than received theories warranted.
Here, then, was a "residual phenomenon" of the utmost promise for the
disclosure of novel truths. Encke (in accordance with the opinion of
Olbers) explained it as due to the presence in space of some such
"subtle matter" as was long ago invoked by Euler[247] to be the agent of
eventual destruction for the fair scheme of planetary creation. The
apparent anomaly of accounting for an accelerative effect by a retarding
cause disappears when it is considered that any check to the motion of
bodies revolving round a centre of attraction causes them to draw closer
to it, thus shortening their periods and quickening their circulation.
If space were filled with a resisting medium capable of impeding, even
in the most infinitesimal degree, the swift course of the planets, their
orbits should necessarily be, not ellipses, but very close elliptical
spirals along which they would slowly, but inevitably, descend into the
burning lap of the sun. The circumstance that no such tendency can be
traced in their revolutions by no means sets the question at rest. For
it might well be that an effect totally imperceptible until after the
lapse of countless ages, as regards the solid orbs of our system, might
be obvious in the movements of bodies like comets of small mass and
great bulk; just as a feather or a gauze veil at once yields its motion
to the resistance of the air, while a cannon-ball cuts its way through
with comparatively slight loss of velocity.
It will thus be seen that issues of the most momentous character hang on
the _time-keeping_ of comets; for plainly all must in some degree suffer
the same kind of hindrance as Encke's, if the cause of that hindrance be
the one suggested. None of its congeners, however, show any trace of
similar symptoms. True, the late Professor Oppolzer announced,[248] in
1880, that a comet, first seen by Pons in 1819, and rediscovered by
Winnecke in 1858, having a period of 2,052 days (5·6 years), was
accelerated at each revolution precisely in the manner required by
Encke's theory. But M. von Haerdtl's subsequent investigation, the
materials for which included numerous observations of the body in
question at its return to the sun in 1886, decisively negatived the
presence of any such effect.[249] Moreover, the researches of Von Asten
and Backlund[250] into the movements of Encke's comet revealed a
perplexing circumstance. They confirmed Encke's results for the period
covered by them, but exhibited the acceleration as having _suddenly
diminished_ by nearly one-half in 1868. The reality and permanence of
this change were fully established by observations of the ensuing return
in March, 1885. Some physical alteration of the retarded body seems
indicated; but visual evidence countenances no such assumption. In
aspect the comet is no less thin and diffuse than in 1795 or in 1848.
The character of the supposed resistance in inter-planetary space has,
it may be remarked, been often misapprehended. What Encke stipulated for
was not a medium equally diffused throughout the visible universe, such
as the ethereal vehicle of the vibrations of light, but a rare fluid,
rapidly increasing in density towards the sun.[251] This cannot be a
solar atmosphere, since it is mathematically certain, as Laplace has
shown,[252] that no envelope partaking of the sun's axial rotation can
extend farther from his surface than nine-tenths of the mean distance of
Mercury; while physical evidence assures us that the _actual_ depth of
the solar atmosphere bears a very minute proportion to the _possible_
depth theoretically assigned to it. That matter, however, not
atmospheric in its nature--that is, neither forming one body with the
sun nor altogether aëriform--exists in its neighbourhood, can admit of
no reasonable doubt. The great lens-shaped mass of the zodiacal light,
stretching out at times far beyond the earth's orbit, may indeed be
regarded as an extension of the corona, the streamers of which
themselves mark the wide diffusion, all round the solar globe, of
granular or gaseous materials. Yet comets have been known to penetrate
the sphere occupied by them without perceptible loss of velocity. The
hypothesis, then, of a resisting medium receives at present no
countenance from the movements of comets, whether of short or of long
periods.
Although Encke's comet has made thirty-five complete rounds of its orbit
since its first detection in 1786, it shows no certain signs of decay.
Variations in its brightness are, it is true, conspicuous, but they do
not proceed continuously.[253]
The history of the next known planet-like comet has proved of even more
curious interest than that of the first. It was discovered by an
Austrian officer named Wilhelm von Biela at Josephstadt in Bohemia,
February 27, 1826, and ten days later by the French astronomer Gambart
at Marseilles. Both observers computed its orbit, showed its remarkable
similarity to that traversed by comets visible in 1772 and 1805, and
connected them together as previous appearances of the body just
detected by assigning to its revolutions a period of between six and
seven years. The two brief letters conveying these strikingly similar
inferences were printed side by side in the same number of the
_Astronomische Nachrichten_ (No. 94); but Biela's priority in the
discovery of the comet was justly recognised by the bestowal upon it of
his name.
The object in question was at no time, subsequently to 1805, visible to
the naked eye. Its aspect in Sir John Herschel's great reflector on the
23rd of September, 1832, was described by him as that of a "conspicuous
nebula," nearly 3 minutes in diameter. No trace of a tail was
discernible. While he was engaged in watching it, a small knot of minute
stars was directly traversed by it, "and when on the cluster," he tells
us,[254] it "presented the appearance of a nebula resolvable and partly
resolved into stars, the stars of the cluster being visible through the
comet." Yet the depth of cometary matter through which such faint
stellar rays penetrated undimmed, was, near the central parts of the
globe, not less than 50,000 miles.
It is curious to find that this seemingly harmless, and we may perhaps
add effete body, gave occasion to the first (and not the last) cometary
"scare" of an enlightened century. Its orbit, at the descending node,
may be said to have intersected that of the earth; since, according as
it _bulged in or out_ under the disturbing influence of the planets, the
passage of the comet was affected _inside_ or _outside_ the terrestrial
track. Now, certain calculations published by Olbers in 1828[255] showed
that, on October 29, 1832, a considerable portion of its nebulous
surroundings would actually sweep over the spot which, a month later,
would be occupied by our planet. It needed no more to set the popular
imagination in a ferment. Astronomers, after all, could not, by an
alarmed public, be held to be infallible. Their computations, it was
averred, which a trifling oversight would suffice to vitiate, exhibited
clearly enough the danger, but afforded no guarantee of safety from a
collision, with all the terrific consequences frigidly enumerated by
Laplace. Nor did the panic subside until Arago formally demonstrated
that the earth and the comet could by no possibility approach within
less than fifty millions of miles.[256]
The return of the same body in 1845-46 was marked by an extraordinary
circumstance. When first seen, November 28, it wore its usual aspect of
a faint round patch of cosmical fog; but on December 19, Mr. Hind
noticed that it had become distorted somewhat into the form of a pear;
and ten days later, it had divided into two separate objects. This
singular duplication was first perceived at New Haven in America,
December 29,[257] by Messrs. Herrick and Bradley, and by Lieutenant
Maury at Washington, January 13, 1846. The earliest British observer of
the phenomenon (noticed by Wichmann the same evening at Königsberg) was
Professor Challis. "I see _two_ comets!" he exclaimed, putting his eye
to the great equatoreal of the Cambridge Observatory on the night of
January 15; then, distrustful of what his senses had told him, he called
in his judgment to correct their improbable report by resolving one of
the dubious objects into a hazy star.[258] On the 23rd, however, both
were again seen by him in unmistakable cometary shape, and until far on
in March (Otto Struve caught a final glimpse of the pair on the 16th of
April),[259] continued to be watched with equal curiosity and amazement
by astronomers in every part of the northern hemisphere. What Seneca
reproved Ephorus for supposing to have taken place in 373 b.c.--what
Pingré blamed Kepler for conjecturing in 1618--had then actually
occurred under the attentive eyes of science in the middle of the
nineteenth century!
At a distance from each other of about two-thirds the distance of the
moon from the earth, the twin comets meantime moved on tranquilly, so
far, at least, as their course through the heaven was concerned. Their
extreme _lightness_, or the small amount of matter contained in each,
could not have received a more signal illustration than by the fact that
their revolutions round the sun were performed independently; that is to
say, they travelled side by side without experiencing any appreciable
mutual disturbance, thus plainly showing that at an interval of only
157,250 miles their attractive power was virtually inoperative. Signs of
internal agitation, however, were not wanting. Each fragment threw out a
short tail in a direction perpendicular to the line joining their
centres, and each developed a bright nucleus, although the original
comet had exhibited neither of these signs of cometary vitality. A
singular interchange of brilliancy was, besides, observed to take place
between the coupled objects, each of which alternately outshone and was
outshone by the other, while an arc of light, apparently proceeding from
the more lustrous, at times bridged the intervening space. Obviously,
the gravitational tie, rendered powerless by exiguity of matter, was
here replaced by some other form of mutual action, the nature of which
can as yet be dealt with only by conjecture.
Once more, in August, 1852, the double comet returned to the
neighbourhood of the sun, but under circumstances not the most
advantageous for observation. Indeed, the companion was not detected
until September 16, when Father Secchi at Rome perceived it to have
increased its distance from the originating body to a million and a
quarter of miles, or about eight times the average interval at the
former appearance. Both vanished shortly afterwards, and have never
since been seen, notwithstanding the eager watch kept for objects of
such singular interest, and the accurate knowledge of their track
supplied by Santini's investigations. A dangerously near approach to
Jupiter in 1841 is believed to have occasioned their disruption, and the
disaggregating process thus started was likely to continue. We can
scarcely doubt that the fate has overtaken them which Newton assigned as
the end of all cometary existence. _Diffundi tandem et spargi per
coelos universos._[260]
Biela's is not the only vanished comet. Brorsen's, discovered at Kiel in
1846, and observed at four subsequent returns, failed unaccountably to
become visible in 1890.[261] Yet numerous sentinels were on the alert to
surprise its approach along a well-ascertained track, traversed in five
and a half years. The object presented from the first a somewhat
time-worn aspect. It was devoid of tail, or any other kind of appendage;
and the rapid loss of the light acquired during perihelion passage was
accompanied by inordinate expansion of an already tenuous globular mass.
Another lost or mislaid comet is one found by De Vico at Rome, August
22, 1844. It was expected to return early in 1850, but did not, and has
never since been seen; unless its re-appearance as E. Swift's comet of
1894 should be ratified by closer inquiry.[262]
A telescopic comet with a period of 7-1/2 years, discovered November 22,
1843, by M. Faye of the Paris Observatory, formed the subject of a
characteristically patient and profound inquiry on the part of
Leverrier, designed to test its suggested identity with Lexell's comet
of 1770. The result was decisive against the hypothesis of Valz, the
divergences between the orbits of the two bodies being found to increase
instead of to diminish, as the history of the new-comer was traced
backward into the previous century.[263] Faye's comet pursues the most
nearly circular path of any similar known object; even at its nearest
approach to the sun it remains farther off than Mars when he is most
distant from it; and it was proved by the admirable researches of
Professor Axel Möller,[264] director of the Swedish observatory of Lund,
to exhibit no trace of the action of a resisting medium.
Periodical comets are evidently bodies which have each lived through a
chapter of accidents, and a significant hint as to the nature of their
adventures can be gathered from the fact that their aphelia are pretty
closely grouped about the tracks of the major planets. Halley's, and
five other comets are thus related to Neptune; three connect themselves
with Uranus, two with Saturn, above a score with Jupiter. Some form of
dependence is plainly indicated, and the researches of Tisserand,[265]
Callandreau,[266] and Newton[267] of Yale College, leave scarcely a
doubt that the "capture-theory" represents the essential truth in the
matter. The original parabolic paths of these comets were then changed
into ellipses by the backward pull of a planet, whose sphere of
attraction they chanced to enter when approaching the sun from outer
space. Moreover, since a body thus affected should necessarily return at
each revolution to the scene of encounter, the same process of
retardation may, in some cases, have been repeated many times, until the
more restricted cometary orbits were reduced to their present
dimensions. The prevalence, too, among periodical comets, of direct
motion, is shown to be inevitable by M. Callandreau's demonstration that
those travelling in a retrograde direction would, by planetary action,
be thrown outside the probable range of terrestrial observation. The
scarcity of hyperbolic comets can be similarly explained. They would be
created whenever the attractive influence of the disturbing planet was
exerted in a forward or accelerative sense, but could come only by a
rare exception to our notice. The inner planets, including the earth,
have also unquestionably played their parts in modifying cometary
orbits; and Mr. Plummer suggests, with some show of reason, that the
capture of Encke's comet may be a feat due to Mercury.[268]
No _great_ comet appeared between the "star" which presided at the birth
of Napoleon and the "vintage" comet of 1811. The latter was first
described by Flaugergues at Viviers, March 26, 1811; Wisniewski, at
Neu-Tscherkask in Southern Russia, caught a final glimpse of it, August
17, 1812. Two disappearances in the solar rays as the earth moved round
in its orbit, and two reappearances after conjunction, were included in
this unprecedentedly long period of visibility of 510 days. This
relative permanence (so far as the inhabitants of Europe were concerned)
was due to the high northern latitude attained near perihelion, combined
with a certain leisureliness of movement along a path everywhere
external to that of the earth. The magnificent luminous train of this
body, on October 15, the day of its nearest terrestrial approach,
covered an arc of the heavens 23-1/2 degrees in length, corresponding to
a real extension of one hundred millions of miles. Its form was
described by Sir William Herschel as that of "an inverted hollow cone,"
and its colour as yellowish, strongly contrasted with the bluish-green
tint of the "head," round which it was flung like a transparent veil.
The planetary disc of the head, 127,000 miles across, appeared to be
composed of strongly-condensed nebulous matter; but somewhat
eccentrically situated within it was a star-like nucleus of a reddish
tinge, which Herschel presumed to be solid, and ascertained, with his
usual care, to have a diameter of 428 miles. From the total absence of
phases, as well as from the vivacity of its radiance, he confidently
inferred that its light was not borrowed, but inherent.[269]
This remarkable apparition formed the subject of a memoir by
Olbers,[270] the striking yet steadily reasoned out suggestions
contained in which there was at that time no means of following up with
profit. Only of late has the "electrical theory," of which Zöllner[271]
regarded Olbers as the founder, assumed a definite and measurable form,
capable of being tested by the touchstone of fact, as knowledge makes
its slow inroads on the fundamental mystery of the physical universe.
The paraboloidal shape of the bright envelope separated by a dark
interval from the head of the great comet of 1811, and constituting, as
it were, the _root_ of its tail, seemed to the astronomer of Bremen to
reveal the presence of a double repulsion; the expelled vapours
accumulating where the two forces, solar and cometary, balanced each
other, and being then swept backwards in a huge train. He accordingly
distinguished three classes of these bodies:--First, comets which
develop _no_ matter subject to solar repulsion. These have no tails, and
are probably mere nebulosities, without solid nuclei. Secondly, comets
which are acted upon by solar repulsion _only_, and consequently throw
out no emanations _towards_ the sun. Of this kind was a bright comet
visible in 1807.[272] Thirdly, comets like that of 1811, giving evidence
of action of both kinds. These are distinguished by a dark _hoop_
encompassing the head and dividing it from the luminous envelope, as
well as by an obscure caudal axis, resulting from the hollow, cone-like
structure of the tail.
Again, the ingenious view subsequently propounded by M. Bredikhin as to
the connection between the _form_ of these appendages and the _kind_ of
matter composing them, was very clearly anticipated by Olbers. The
amount of tail-curvature, he pointed out, depends in each case upon the
proportion borne by the velocity of the ascending particles to that of
the comet in its orbit; the swifter the outrush, the straighter the
resulting tail. But the velocity of the ascending particles varies with
the energy of their repulsion by the sun, and this again, it may be
presumed, with their quality. Thus multiple tails are developed when the
same comet throws off, as it approaches perihelion, specifically
distinct substances. The long, straight ray which proceeded from the
comet of 1807, for example, was doubtless made up of particles subject
to a much more vigorous solar repulsion than those formed into the
shorter curved emanation issuing from it nearly in the same direction.
In the comet of 1811 he calculated that the particles expelled from the
head travelled to the remote extremity of the tail in eleven minutes,
indicating by this enormous rapidity of movement (comparable to that of
the transmission of light) the action of a force much more powerful than
the opposing one of gravity. The not uncommon phenomena of multiple
envelopes, on the other hand, he explained as due to the varying amounts
of repulsion exercised by the nucleus itself on the different kinds of
matter developed from it.
The movements and perturbations of the comet of 1811 were no less
profoundly studied by Argelander than its physical constitution by
Olbers. The orbit which he assigned to it is of such vast dimensions as
to require no less that 3,065 years for the completion of its circuit;
and to carry the body describing it at each revolution to fourteen times
the distance from the sun of the frigid Neptune. Thus, when it last
visited our neighbourhood, Achilles may have gazed on its imposing train
as he lay on the sands all night bewailing the loss of Patroclus; and
when it returns, it will perhaps be to shine upon the ruins of empires
and civilizations still deep buried among the secrets of the coming
time.[273]
On the 26th of June, 1819, while the head of a comet passed across the
face of the sun, the earth was in all probability involved in its tail.
But of this remarkable double event nothing was known until more than a
month later, when the fact of its past occurrence emerged from the
calculations of Olbers.[274] Nor had the comet itself been generally
visible previous to the first days of July. Several observers, however,
on the publication of these results, brought forward accounts of
singular spots perceived by them upon the sun at the time of the
transit, and an original drawing of one of them, by Pastorff of
Buchholtz, has been preserved. This undoubtedly authentic
delineation[275] represents a round nebulous object with a _bright_ spot
in the centre, of decidedly cometary aspect, and not in the least like
an ordinary solar "macula." Mr. Hind,[276] nevertheless, showed its
position on the sun to be irreconcilable with that which the comet must
have occupied; and Mr. Ranyard's discovery of a similar smaller drawing
by the same author, dated May 26, 1828,[277] reduces to evanescence the
probability of its connection with that body. Indeed, recent experience
renders very doubtful the possibility of such an observation.
The return of Halley's comet in 1835 was looked forward to as an
opportunity for testing the truth of floating cometary theories, and did
not altogether disappoint expectation. As early as 1817, its movements
and disturbances since 1759 were proposed by the Turin Academy of
Sciences as the subject of a prize ultimately awarded to Baron
Damoiseau. Pontécoulant was adjudged a similar distinction by the Paris
Academy in 1829; while Rosenberger's calculations were rewarded with the
gold medal of the Royal Astronomical Society.[278]
They were verified by the detection at Rome, August 6, 1835, of a nearly
circular misty object not far from the predicted place of the comet. It
was not, however, until the middle of September that it began to throw
out a tail, which by the 15th of October had attained a length of about
24 degrees (on the 19th, at Madras, it extended to fully 30),[279] the
head showing to the naked eye as a reddish star rather brighter than
Aldebaran or Antares.[280] Some curious phenomena accompanied the
process of tail-formation. An outrush of luminous matter, resembling in
shape a partially opened fan, issued from the nucleus _towards_ the sun,
and at a certain point, like smoke driven before a high wind, was
vehemently swept backwards in a prolonged train. The appearance of the
comet at this time was compared by Bessel,[281] who watched it with
minute attention, to that of a blazing rocket. He made the singular
observation that this fan of light, which seemed the source of supply
for the tail, oscillated like a pendulum to and fro across a line
joining the sun and nucleus, in a period of 4-3/5 days; and he was
unable to escape from the conclusion[282] that a repulsive force, about
twice as powerful as the attractive force of gravity, was concerned in
the production of these remarkable effects. Nor did he hesitate to recur
to the analogy of magnetic polarity, or to declare, still more
emphatically than Olbers, "the emission of the tail to be a purely
electrical phenomenon."[283]
The transformations undergone by this body were almost as strange and
complete as those which affected the brigands in Dante's _Inferno_. When
first seen, it wore the aspect of a nebula; later it put on the
distinctive garb of a comet; it next appeared as a star; finally, it
dilated, first in a spherical, then in a paraboloidal form, until May 5,
1836, when it vanished from Herschel's observation at Feldhausen as if
by melting into adjacent space from the excessive diffusion of its
light. A very uncommon circumstance in its development was that it lost
all trace of tail _previous_ to its arrival at perihelion on the 16th of
November. Nor did it begin to recover its elongated shape for more than
two months afterwards. On the 23rd of January, Boguslawski perceived it
as a star of the sixth magnitude, _without measurable disc_.[284] Only
two nights later, Maclear, director of the Cape Observatory, found the
head to be 131 seconds across.[285] And so rapidly did the augmentation
of size progress, that Sir John Herschel estimated the actual bulk of
this singular object to have increased forty-fold in the ensuing week.
"I can hardly doubt," he remarks, "that the comet was fairly evaporated
in perihelio by the heat, and resolved into transparent vapour, and is
now in process of rapid condensation and re-precipitation on the
nucleus."[286] A plausible, but no longer admissible, interpretation of
this still unexplained phenomenon. The next return of this body, which
will be considerably accelerated by Jupiter's influence, is expected to
take place in 1910.[287]
By means of an instrument devised to test the quality of light, Arago
obtained decisive evidence that some at least of the radiance proceeding
from Halley's comet was derived by reflection from the sun.[288]
Indications of the same kind had been afforded[289] by the comet which
suddenly appeared above the north-western horizon of Paris, July 3,
1819, after having enveloped (as already stated) our terrestrial abode
in its filmy appendages; but the "polariscope" had not then reached the
perfection subsequently given to it, and its testimony was accordingly
far less reliable than in 1835. Such experiments, however, are in
reality more beautiful and ingenious than instructive, since ignited as
well as obscure bodies possess the power of throwing back light incident
upon them, and will consequently transmit to us from the neighbourhood
of the sun rays partly direct, partly reflected, of which a certain
proportion will exhibit the peculiarity known as polarisation.
The most brilliant comets of the century were suddenly rivalled if not
surpassed by the extraordinary object which blazed out beside the sun,
February 28, 1843. It was simultaneously perceived in Mexico and the
United States, in Southern Europe, and at sea off the Cape of Good Hope,
where the passengers on board the _Owen Glendower_ were amazed by the
sight of a "short, dagger-like object," closely following the sun
towards the western horizon.[290] At Florence, Amici found its distance
from the sun's centre at noon to be only 1° 23'; and spectators at Parma
were able, when sheltered from the direct glare of mid-day, to trace the
tail to a length of four or five degrees. The full dimensions of this
astonishing appurtenance began to be disclosed a few days later. On the
3rd of March it measured 25°, and on the 11th, at Calcutta, Mr. Clerihew
observed a second streamer, nearly twice as long as the first, and
making an angle with it of 18°, to have been emitted in a single day.
This rapidity of projection, Sir John Herschel remarked, "conveys an
astounding impression of the intensity of the forces at work." "It is
clear," he continued, "that _if we have to deal here with matter, such
as we conceive it_--viz., _possessing inertia--at all_, it must be under
the dominion of forces incomparably more energetic than gravitation, and
quite of a different nature."[291]
On the 17th of March a silvery ray, some 40° long and slightly curved at
its extremity, shone out above the sunset clouds in this country. No
previous intimation had been received of the possibility of such an
apparition, and even astronomers--no lightning messages across the seas
being as yet possible--were perplexed. The nature of the phenomenon,
indeed, soon became evident, but the wonder of it did not diminish with
the study of its attendant circumstances. Never before, within
astronomical memory, had our system been traversed by a body pursuing
such an adventurous career. The closest analogy was offered by the great
comet of 1680 (Newton's), which rushed past the sun at a distance of
only 144,000 miles; but even this--on the cosmical scale--scarcely
perceptible interval was reduced nearly one-half in the case we are now
concerned with. The centre of the comet of 1843 approached the
formidable luminary within 78,000 miles, leaving, it is estimated, a
clear space of not more than 32,000 between the surfaces of the bodies
brought into such perilous proximity. The escape of the wanderer was,
however, secured by the extraordinary rapidity of its flight. It swept
past perihelion at a rate--366 miles a second--which, if continued,
would have carried it right round the sun in _two hours_; and in only
eleven minutes more than that short period it actually described half
the _curvature_ of its orbit--an arc of 180°--although in travelling
over the remaining half many hundreds of sluggish years will doubtless
be consumed.
The behaviour of this comet may be regarded as an _experimentum crucis_
as to the nature of tails. For clearly no fixed appendage many millions
of miles in length could be whirled like a brandished sabre from one
side of the sun to the other in 131 minutes. Cometary trains are then,
as Olbers rightly conceived them to be, emanations, not
appendages--inconceivably rapid outflows of highly rarefied matter, the
greater part, if not all, of which becomes permanently detached from the
nucleus.
That of the comet of 1843 reached, about the time that it became visible
in this country, the extravagant length of 200 millions of miles.[292]
It was narrow, and bounded by nearly parallel and nearly rectilinear
lines, resembling--to borrow a comparison of Aristotle's--a "road"
through the constellations; and after the 3rd of March showed no trace
of hollowness, the axis being, in fact, rather brighter than the edges.
Distinctly perceptible in it were those singular aurora-like
coruscations which gave to the "tresses" of Charles V.'s comet the
appearance--as Cardan described it--of "a torch agitated by the wind,"
and have not unfrequently been observed to characterise other similar
objects. A consideration first adverted to by Olbers proves these to
originate in our own atmosphere. For owing to the great difference in
the distances from the earth of the origin and extremity of such vast
effluxes, the light proceeding from their various parts is transmitted
to our eyes in notably different intervals of time. Consequently a
luminous undulation, even though propagated instantaneously from end to
end of a comet's tail, would appear to us to occupy many minutes in its
progress. But the coruscations in question pass as swiftly as a falling
star. They are, then, of terrestrial production.
Periods of the utmost variety were by different computators assigned to
the body, which arrived at perihelion, February 27, 1843, at 9.47 p.m.
Professor Hubbard of Washington found that it required 533 years to
complete a revolution; MM. Laugier and Mauvais of Paris considered the
true term to be 35;[293] Clausen looked for its return at the end of
between six and seven. A recent discussion[294] by Professor Kreutz of
all the available data gives a probable period of 512 years for this
body, and precludes its hypothetical identity with the comet of 1668,
known as the "Spina" of Cassini.
It may now be asked, what were the conclusions regarding the nature of
comets drawn by astronomers from the considerable amount of novel
experience accumulated during the first half of this century? The first
and best assured was that the matter composing them is in a state of
extreme tenuity. Numerous and trustworthy observations showed that the
feeblest rays of light might traverse some hundreds of thousands of
miles of their substance, even where it was apparently most condensed,
without being perceptibly weakened. Nay, instances were recorded in
which stars were said to have gained in brightness from the
process![295] On the 24th of June, 1825, Olbers[296] saw the comet then
visible all but obliterated by the central passage of a star too small
to be distinguished with the naked eye, its own light remaining wholly
unchanged. A similar effect was noted December 1, 1811, when the great
comet of that year approached so close to Altair, the _lucida_ of the
Eagle, that the star seemed to be transformed into the nucleus of the
comet.[297] Even the central blaze of Halley's comet in 1835 was
powerless to impede the passage of stellar rays. Struve[298] observed at
Dorpat, on September 17, an all but central occultation; Glaisher[299]
one (so far as he could ascertain) absolutely so eight days later at
Cambridge. In neither case was there any appreciable diminution of the
star's light. Again, on the 11th of October, 1847, Mr. Dawes,[300] an
exceptionally keen observer, distinctly saw a star of the tenth
magnitude through the exact centre of a comet discovered on the first of
that month by Maria Mitchell of Nantucket.
Examples, on the other hand, are not wanting of the diminution of
stellar light under similar circumstances;[301] and we meet two alleged
instances of the vanishing of a star behind a comet. Wartmann of Geneva
observed the first, November 28, 1828;[302] but his instrument was
defective, and the eclipsing body, Encke's comet, has shown itself
otherwise perfectly translucent. The second case of occultation occurred
September 13, 1890, when an eleventh magnitude star was stated to have
completely disappeared during the transit over it of Denning's
comet.[303]
From the failure to detect any effects of refraction in the light of
stars occulted by comets, it was inferred (though, as we know now,
erroneously) that their composition is rather that of dust than that of
vapour; that they consist not of any continuous substance, but of
discrete solid particles, very finely divided and widely scattered. In
conformity with this view was the known smallness of their masses.
Laplace had shown that if the amount of matter forming Lexell's comet
had been as much as 1/5000 of that contained in our globe, the effect of
its attraction, on the occasion of its approach within 1,438,000 miles
of the earth, July 1, 1770, must have been apparent in the lengthening
of the year. And that some comets, at any rate, possess masses
immeasurably below this maximum value was clearly proved by the
undisturbed parallel march of the two fragments of Biela's in 1846.
But the discovery in this branch most distinctive of the period under
review is that of "short period" comets, of which four[304] were known
in 1850. These, by the character of their movements, serve as a link
between the planetary and cometary worlds, and by the nature of their
construction, seem to mark a stage in cometary decay. For that comets
are rather transitory agglomerations, than permanent products of
cosmical manufacture, appeared to be demonstrated by the division and
disappearance of one amongst their number, as well as by the singular
and rapid changes in appearance undergone by many, and the seemingly
irrevocable diffusion of their substance visible in nearly all. They
might then be defined, according to the ideas respecting them prevalent
fifty years ago, as bodies unconnected by origin with the solar system,
but encountered, and to some extent appropriated, by it in its progress
through space, owing their visibility in great part, if not altogether,
to light reflected from the sun, and their singular and striking forms
to the action of repulsive forces emanating from him, the penalty of
their evanescent splendour being paid in gradual waste and final
dissipation and extinction.
FOOTNOTES:
[Footnote 241: _Allgemeine Geographische Ephemeriden_, vol. iv., p.
287.]
[Footnote 242: _Astr. Jahrbuch_, 1823, p. 217. The period (1,208 days)
of this body is considerably shorter than that of any other known
comet.]
[Footnote 243: "Sicut bombyces filo fundendo, sic cometas cauda
exspiranda consumi et denique mori."--_De Cometis_, Op., vol. vii., p.
110.]
[Footnote 244: Considerable uncertainty, however, still prevails on the
point. The inverse relation assumed by Lagrange to exist between
distance from the sun and density brought out the Mercurian mass
1/2025810 that of the sun (Laplace, _Exposition du Syst. du Monde_, t.
ii., p. 50, ed. 1824). Von Asten deduced from the movements of Encke's
comet, 1818-48, a value of 1/7636440; while Backlund from its seven
returns, 1871-1891, derived 1/9647000 (_Comptes Rendus_, Oct. 1, 1894).]
[Footnote 245: Arago, _Annuaire_ (1832), p. 218.]
[Footnote 246: Hind, _The Comets_, p. 20.]
[Footnote 247: _Phil. Trans._, vol. xlvi., p. 204.]
[Footnote 248: _Astr. Nach._, No. 2,134.]
[Footnote 249: _Comptes Rendus_, t. cvii., p. 588.]
[Footnote 250: _Mém. de St. Pétersbourg_, t. xxxii., No. 3, 1884; _Astr.
Nach._, No. 2,727.]
[Footnote 251: _Month. Not._, vol. xix., p. 72.]
[Footnote 252: _Mécanique Céleste_, t. ii., p. 197.]
[Footnote 253: See Berberich, _Astr. Nach._, Nos. 2,836-7, 3,125;
Deichmüller, _Ibid._, No. 3,123.]
[Footnote 254: _Month. Not._, vol. ii., p. 117.]
[Footnote 255: _Astr. Nach._, No. 128.]
[Footnote 256: _Annuaire_, 1832, p. 186.]
[Footnote 257: _Am. Journ. of Science_, vol. i. (2nd series), p. 293.
Prof. Hubbard's calculations indicated a probability that the definitive
separation of the two nuclei occurred as early as September 30, 1884.
_Astronomical Journal_ (Gould's), vol. iv., p. 5. See also, on the
subject of this comet, W. T. Lynn, _Intellectual Observer_, vol. xi., p.
208; E. Ledger, _Observatory_, August, 1883, p. 244; and H. A. Newton,
_Am. Journ. of Science_, vol. xxxi., p. 81, February, 1886.]
[Footnote 258: _Month. Not._, vol. vii., p. 73.]
[Footnote 259: _Bulletin Ac. Imp. de St. Pétersbourg_, t. vi., col. 77.
The latest observation of the parent nucleus was that of Argelander,
April 27, at Bonn.]
[Footnote 260: D'Arrest, _Astr. Nach._, No. 1,624.]
[Footnote 261: _Der Brorsen'sche Comet._ Von Dr. E. Lamp, Kiel, 1892;
Plummer, _Knowledge_, vol. xix., p. 41.]
[Footnote 262: Schulhof, _Astr. Nach._, No. 3,267; _Observatory_, vol.
xviii., p. 64; F. H. Seares, _Astr. Nach._, Nos. 3,606-7; Plummer,
_Knowledge_, vol. xix., p. 156.]
[Footnote 263: _Comptes Rendus_, t. xxv., p. 570.]
[Footnote 264: _Month. Not._, vol. xii., p. 248.]
[Footnote 265: _Bull. Astr._, t. vi., pp. 241, 289.]
[Footnote 266: _Étude sur la Théorie des Comètes périodiques. Annales de
l'Observatoíre_, t. xx., Paris, 1891.]
[Footnote 267: _Amer. Journ. of Science_, vol. xlii., pp. 183, 482,
1891.]
[Footnote 268: _Observatory_, vol. xiv., p. 194.]
[Footnote 269: _Phil. Trans._, vol. cii., pp. 118-124.]
[Footnote 270: _Ueber den Schweif des grossen Cometen von 1811, Monat.
Corr._, vol. xxv., pp. 3-22. Reprinted by Zöllner. _Ueber die Natur der
Cometen_, pp. 3-15.]
[Footnote 271: _Natur der Cometen_, p. 148.]
[Footnote 272: The subject of a classical memoir by Bessel, published in
1810.]
[Footnote 273: A fresh investigation of its orbit has been published by
N. Herz of Vienna. See _Bull. Astr._, t. ix., p. 427.]
[Footnote 274: _Astr. Jahrbuch_ (Bode's), 1823, p. 134.]
[Footnote 275: Reproduced in Webb's _Celestial Objects_, 4th ed.]
[Footnote 276: _Month. Not._, vol. xxxvi., p. 309.]
[Footnote 277: _Celestial Objects_, p. 40, note.]
[Footnote 278: See Airy's Address, _Mem. R. A. S._, vol. x., p. 376.
Rosenberger calculated no more, though he lived until 1890. W. T. Lynn,
_Observatory_, vol. xiii., p. 125.]
[Footnote 279: Hind, _The Comets_, p. 47.]
[Footnote 280: Arago, _Annuaire_, 1836, p. 228.]
[Footnote 281: _Astr. Nach._, No. 300.]
[Footnote 282: It deserves to be recorded that Robert Hooke drew a very
similar inference from his observations of the comets of 1680 and 1682.
_Month. Not._, vol. xiv., pp. 77-83.]
[Footnote 283: _Briefwechsel zwischen Olbers und Bessel_, Bd. ii., p.
390.]
[Footnote 284: Herschel, _Results_, p. 405.]
[Footnote 285: _Mem. R. A. S._, vol. x., p. 92,]
[Footnote 286: _Results_, p. 401.]
[Footnote 287: Pontécoulant, _Comptes Rendus_, t. lviii., p. 825.]
[Footnote 288: _Annuaire_, 1836, p. 233.]
[Footnote 289: _Cosmos_, vol. i., p. 90, _note_ (Otté's trans.).]
[Footnote 290: Herschel, _Outlines of Astronomy_, p. 399, 9th ed.]
[Footnote 291: _Outlines_, p. 398.]
[Footnote 292: Boguslawski calculated that it extended on the 21st of
March to 581 millions.--_Report. Brit. Ass._, 1845, p. 89.]
[Footnote 293: _Comptes Rendus_, t. xvi., p. 919.]
[Footnote 294: _Observatory_, vol. xxiv., p. 167; Astr. Nach., No.
3,320.]
[Footnote 295: Piazzi noticed a considerable increase of lustre in a
very faint star of the twelfth magnitude viewed through a comet. Mädler,
_Reden_, etc., p. 248, _note_.]
[Footnote 296: _Astr. Jahrbuch_, 1828, p. 151.]
[Footnote 297: Mädler, _Gesch. d. Astr._, Bd. ii., p. 412.]
[Footnote 298: _Recueil de l'Ac. Imp. de St. Pétersbourg_, 1835, p.
143.]
[Footnote 299: Guillemin's _World of Comets_, trans, by J. Glaisher, p.
294, _note_.]
[Footnote 300: _Month. Not._, vol. viii., p. 9.]
[Footnote 301: A real, though only partial stoppage of light seems
indicated by Herschel's observations on the comet of 1807. Stars seen
through the tail, October 18, lost much of their lustre. One near the
head was only faintly visible by glimpses. _Phil. Trans._, vol. xcvii.,
p. 153.]
[Footnote 302: Arago, _Annuaire_, 1832, p. 205.]
[Footnote 303: _Ibid._, 1891, p. 290.]
[Footnote 304: Viz., Encke's, Biela's, Faye's, and Brorsen's.]
CHAPTER VI
_INSTRUMENTAL ADVANCES_
It is impossible to follow with intelligent interest the course of
astronomical discovery without feeling some curiosity as to the means by
which such surpassing results have been secured. Indeed, the bare
acquaintance with _what_ has been achieved, without any corresponding
knowledge of _how_ it has been achieved, supplies food for barren wonder
rather than for fruitful and profitable thought. Ideas advance most
readily along the solid ground of practical reality, and often find true
sublimity while laying aside empty marvels. Progress is the result, not
so much of sudden flights of genius, as of sustained, patient, often
commonplace endeavour; and the true lesson of scientific history lies in
the close connection which it discloses between the most brilliant
developments of knowledge and the faithful accomplishment of his daily
task by each individual thinker and worker.
It would be easy to fill a volume with the detailed account of the long
succession of optical and mechanical improvements by means of which the
observation of the heavens has been brought to its present degree of
perfection; but we must here content ourselves with a summary sketch of
the chief amongst them. The first place in our consideration is
naturally claimed by the telescope.
This marvellous instrument, we need hardly remind our readers, is of two
distinct kinds--that in which light is gathered together into a focus by
_refraction_, and that in which the same end is attained by
_reflection_. The image formed is in each case viewed through a
magnifying lens, or combination of lenses, called the eye-piece. Not for
above a century after the "optic glasses" invented or stumbled upon by
the spectacle-maker of Middelburg (1608) had become diffused over
Europe, did the reflecting telescope come, even in England, the place of
its birth, into general use. Its principle (a sufficiently obvious one)
had indeed been suggested by Mersenne as early as 1639;[305] James
Gregory in 1663[306] described in detail a mode of embodying that
principle in a practical shape; and Newton, adopting an original system
of construction, actually produced in 1668 a tiny speculum, one inch
across, by means of which the apparent distance of objects was reduced
thirty-nine times. Nevertheless, the exorbitantly long tubeless
refractors, introduced by Huygens, maintained their reputation until
Hadley exhibited to the Royal Society, January 12, 1721,[307] a
reflector of six inches aperture, and sixty-two in focal length, which
rivalled in performance, and of course indefinitely surpassed in
manageability, one of the "aerial" kind of 123 feet.
The concave-mirror system now gained a decided ascendant, and was
brought to unexampled perfection by James Short of Edinburgh during the
years 1732-68. Its resources were, however, first fully developed by
William Herschel. The energy and inventiveness of this extraordinary man
marked an epoch wherever they were applied. His ardent desire to measure
and gauge the stupendous array of worlds which his specula revealed to
him, made him continually intent upon adding to their "space-penetrating
power" by increasing their light-gathering surface. These, as he was the
first to explain,[308] are in a constant proportion one to the other.
For a telescope with twice the linear aperture of another will collect
four times as much light, and will consequently disclose an object four
times as faint as could be seen with the first, or, what comes to the
same, an object equally bright at twice the distance. In other words, it
will possess double the space-penetrating power of the smaller
instrument. Herschel's great mirrors--the first examples of the giant
telescopes of modern times--were then primarily engines for extending
the bounds of the visible universe; and from the sublimity of this
"final cause" was derived the vivid enthusiasm which animated his
efforts to success.
It seems probable that the seven-foot telescope constructed by him in
1775--that is within little more than a year after his experiments in
shaping and polishing metal had begun--already exceeded in effective
power any work by an earlier optician; and both his skill and his
ambition rapidly developed. His efforts culminated, after mirrors of
ten, twenty, and thirty feet focal length had successively left his
hands, in the gigantic forty-foot, completed August 28, 1789. It was the
first reflector in which only a single mirror was employed. In the
"Gregorian" form, the focussed rays are, by a second reflection from a
small concave[309] mirror, thrown _straight back_ through a central
aperture in the larger one, behind which the eye-piece is fixed. The
object under examination is thus seen in the natural direction. The
"Newtonian," on the other hand, shows the object in a line of sight at
right angles to the true one, the light collected by the speculum being
diverted to one side of the tube by the interposition of a small plane
mirror, situated at an angle of 45° to the axis of the instrument. Upon
these two systems Herschel worked until 1787, when, becoming convinced
of the supreme importance of economising light (necessarily wasted by
the second reflection), he laid aside the small mirror of his forty-foot
then in course of construction, and turned it into a "front-view"
reflector. This was done--according to the plan proposed by Lemaire in
1732--by slightly inclining the speculum so as to enable the image
formed by it to be viewed with an eye-glass fixed at the upper margin of
the tube. The observer thus stood with his back turned to the object he
was engaged in scrutinising.
The advantages of the increased brilliancy afforded by this modification
were strikingly illustrated by the discovery, August 28 and September
17, 1789, of the two Saturnian satellites nearest the ring.
Nevertheless, the monster telescope of Slough cannot be said to have
realised the sanguine expectations of its constructor. The occasions on
which it could be usefully employed were found to be extremely rare. It
was injuriously affected by every change of temperature. The great
weight (25 cwt.) of a speculum four feet in diameter rendered it
peculiarly liable to distortion. With all imaginable care, the delicate
lustre of its surface could not be preserved longer than two years,[310]
when the difficult process of repolishing had to be undertaken. It was
accordingly never used after 1811, when, having _gone blind_ from damp,
it lapsed by degrees into the condition of a museum inmate.
The exceedingly high magnifying powers employed by Herschel constituted
a novelty in optical astronomy, to which he attached great importance.
The work of ordinary observation would, however, be hindered rather than
helped by them. The attempt to increase in this manner the efficacy of
the telescope is speedily checked by atmospheric, to say nothing of
other difficulties. Precisely in the same proportion as an object is
magnified, the disturbances of the medium through which it is seen are
magnified also. Even on the clearest and most tranquil nights, the air
is never for a moment really still. The rays of light traversing it are
continually broken by minute fluctuations of refractive power caused by
changes of temperature and pressure, and the currents which these
engender. With such luminous quiverings and waverings the astronomer has
always more or less to reckon; their absence is simply a question of
degree; if sufficiently magnified, they are at all times capable of
rendering observation impossible.
Thus, such powers as 3,000, 4,000, 5,000, even 6,652,[311] which
Herschel now and again applied to his great telescopes, must, save on
the rarest occasions, prove an impediment rather than an aid to vision.
They were, however, used by him only for special purposes,
experimentally, not systematically, and with the clearest discrimination
of their advantages and drawbacks. It is obvious that perfectly
different ends are subserved by increasing the _aperture_ and by
increasing the _power_ of a telescope. In the one case, a larger
quantity of light is captured and concentrated; in the other, the same
amount is distributed over a wider area. A diminution of brilliancy in
the image accordingly attends, _coeteris paribus_, upon each
augmentation of its apparent size. For this reason, such faint objects
as nebulæ are most successfully observed with moderate powers applied to
instruments of a great capacity for light, the details of their
structure actually disappearing when highly magnified. With stellar
groups the reverse is the case. Stars cannot be magnified, simply
because they are too remote to have any sensible dimensions; but the
space between them can. It was thus for the purpose of dividing very
close double stars that Herschel increased to such an unprecedented
extent the magnifying capabilities of his instruments; and to this
improvement incidentally the discovery of Uranus, March 13, 1781,[312]
was due. For by the examination with strong lenses of an object which,
even with a power of 227, presented a suspicious appearance, he was able
at once to pronounce its disc to be real, not merely "spurious," and so
to distinguish it unerringly from the crowd of stars amidst which it was
moving.
While the reflecting telescope was astonishing the world by its rapid
development in the hands of Herschel, its unpretending rival was slowly
making its way towards the position which the future had in store for
it. The great obstacle which long stood in the way of the improvement of
refractors was the defect known as "chromatic aberration." This is due
to no other cause than that which produces the rainbow and the
spectrum--the separation, or "dispersion" in their passage through a
refracting medium, of the variously coloured rays composing a beam of
white light. In an ordinary lens there is no common point of
concentration; each colour has its own separate focus; and the resulting
image, formed by the superposition of as many images as there are hues
in the spectrum, is indefinitely terminated with a tinted border,
eminently baffling to exactness of observation.
The extravagantly long telescopes of the seventeenth century were
designed to _avoid_ this evil (as well as another source of indistinct
vision in the spherical shape of lenses); but no attempt to _remedy_ it
was made until an Essex gentleman succeeded, in 1733, in so combining
lenses of flint and crown glass as to produce refraction without
colour.[313] Mr. Chester More Hall was, however, equally indifferent to
fame and profit, and took no pains to make his invention public. The
_effective_ discovery of the achromatic telescope was, accordingly,
reserved for John Dollond, whose method of correcting at the same time
chromatic and spherical aberration was laid before the Royal Society in
1758. Modern astronomy may be said to have been thereby rendered
possible. Refractors have always been found better suited than
reflectors to the ordinary work of observatories. They are, so to speak,
of a more robust, as well as of a more plastic nature. They suffer less
from vicissitudes of temperature and climate. They retain their
efficiency with fewer precautions and under more trying circumstances.
Above all, they co-operate more readily with mechanical appliances, and
lend themselves with far greater facility to purposes of exact
measurement.
A practical difficulty, however, impeded the realisation of the
brilliant prospects held out by Dollond's invention. It was found
impossible to procure flint-glass, such as was needed for optical
use--that is, of perfectly homogeneous quality--except in fragments of
insignificant size. Discs of more than two or three inches in diameter
were of extreme rarity; and the crushing excise duty imposed upon the
article by the financial unwisdom of the Government, both limited its
production, and, by rendering experiments too costly for repetition,
barred its improvement.
Up to this time, Great Britain had left foreign competitors far behind
in the instrumental department of astronomy. The quadrants and circles
of Bird, Cary and Ramsden were unapproached abroad. The reflecting
telescope came into existence and reached maturity on British soil. The
refracting telescope was cured of its inherent vices by British
ingenuity. But with the opening of the nineteenth century, the almost
unbroken monopoly of skill and contrivance which our countrymen had
succeeded in establishing was invaded, and British workmen had to be
content to exchange a position of supremacy for one of at least partial
temporary inferiority.
Somewhat about the time that Herschel set about polishing his first
speculum, Pierre Louis Guinand, a Swiss artisan, living near
Chaux-de-Fonds, in the canton of Neuchâtel, began to grind spectacles
for his own use, and was thence led on to the rude construction of
telescopes by fixing lenses in pasteboard tubes. The sight of an England
achromatic stirred a higher ambition, and he took the first opportunity
of procuring some flint glass from England (then the only source of
supply), with the design of imitating an instrument the full
capabilities of which he was destined to be the humble means of
developing. The English glass proving of inferior quality, he conceived
the possibility, unaided and ignorant of the art as he was, of himself
making better, and spent seven years (1784-90) in fruitless experiments
directed to that end. Failure only stimulated him to enlarge their
scale. He bought some land near Les Brenets, constructed upon it a
furnace capable of melting two quintals of glass, and reducing himself
and his family to the barest necessaries of life, he poured his earnings
(he at this time made bells for repeaters) unstintingly into his
crucibles.[314] His undaunted resolution triumphed. In 1799 he carried
to Paris and there showed to Lalande several discs of flawless crystal
four to six inches in diameter. Lalande advised him to keep his secret,
but in 1805 he was induced to remove to Munich, where he became the
instructor of the immortal Fraunhofer. His return to Les Brenets in 1814
was signalised by the discovery of an ingenious mode of removing
striated portions of glass by breaking and re-soldering the product of
each melting, and he eventually attained to the manufacture of perfect
discs up to 18 inches in diameter. An object-glass for which he had
furnished the material to Cauchoix, procured him, in 1823, a royal
invitation to settle in Paris; but he was no longer equal to the change,
and died at the scene of his labours, February 13 following.
This same lens (12 inches across) was afterwards purchased by Sir James
South, and the first observation made with it, February 13, 1830,
disclosed to Sir John Herschel the sixth minute star in the central
group of the Orion nebula, known as the "trapezium."[315] Bequeathed by
South to Trinity College, Dublin, it was employed at the Dunsink
Observatory by Brünnow and Ball in their investigations of stellar
parallax. A still larger objective (of nearly 14 inches) made of
Guinand's glass was secured in Paris, about the same time, by Mr. Edward
Cooper of Markree Castle, Ireland. The peculiarity of the method
discovered at Les Brenets resided in the manipulation, not in the
quality of the ingredients; the secret, that is to say, was not
chemical, but mechanical.[316] It was communicated by Henry Guinand (a
son of the inventor) to Bontemps, one of the directors of the glassworks
at Choisy-le-Roi, and by him transmitted to Messrs. Chance of
Birmingham, with whom he entered into partnership when the revolutionary
troubles of 1848 obliged him to quit his native country. The celebrated
American opticians, Alvan Clark & Sons, derived from the Birmingham firm
the materials for some of their early telescopes, notably the 19-inch
Chicago and 26-inch Washington equatoreals; but the discs for the great
Lick refractor, and others shaped by them in recent years, have been
supplied by Feil of Paris.
Two distinguished amateurs, meanwhile, were preparing to reassert on
behalf of reflecting instruments their claim to the place of honour in
the van of astronomical discovery. Of Mr. Lassell's specula something
has already been said.[317] They were composed of an alloy of copper and
tin, with a minute proportion of arsenic (after the example of
Newton[318]), and were remarkable for perfection of figure and
brilliancy of surface.
The capabilities of the Newtonian plan were developed still more
fully--it might almost be said to the uttermost--by the enterprise of an
Irish nobleman. William Parsons, known as Lord Oxmantown until 1841,
when, on his father's death, he succeeded to the title of Earl of Rosse,
was born at York, June 17, 1800. His public duties began before his
education was completed. He was returned to Parliament as member for
King's County while still an undergraduate at Oxford, and continued to
represent the same constituency for thirteen years (1821-34). From 1845
until his death, which took place, October 31, 1867, he sat, silent but
assiduous, in the House of Lords as an Irish representative peer; he
held the not unlaborious post of President of the Royal Society from
1849 to 1854; presided over the meeting of the British Association at
Cork in 1843, and was elected Vice-Chancellor of Dublin University in
1862. In addition to these extensive demands upon his time and thoughts,
were those derived from his position as practically the feudal chief of
a large body of tenantry in times of great and anxious responsibility,
to say nothing of the more genial claims of an unstinted hospitality.
Yet, while neglecting no public or private duty, this model nobleman
found leisure to render to science services so conspicuous as to entitle
his name to a lasting place in its annals.
He early formed the design of reaching the limits of the attainable in
enlarging the powers of the telescope, and the qualities of his mind
conspired with the circumstances of his fortune to render the design a
feasible one. From refractors it was obvious that no such vast and rapid
advance could be expected. English glass-manufacture was still in a
backward state. So late as 1839, Simms (successor to the distinguished
instrumentalist Edward Troughton) reported a specimen of crystal
scarcely 7-1/2 inches in diameter, and perfect only over six, to be
unique in the history of English glass-making.[319] Yet at that time the
fifteen-inch achromatic of Pulkowa had already left the workshop of
Fraunhofer's successors at Munich. It was not indeed until 1845, when
the impost which had so long hampered their efforts was removed, that
the optical artists of these islands were able to compete on equal terms
with their rivals on the Continent. In the case of reflectors, however,
there seemed no insurmountable obstacle to an almost unlimited increase
of light-gathering capacity; and it was here, after some unproductive
experiments with fluid lenses, that Lord Oxmantown concentrated his
energies.
He had to rely entirely on his own invention, and to earn his own
experience. James Short had solved the problem of giving to metallic
surfaces a perfect parabolic figure (the only one by which parallel
incident rays can be brought to an exact focus); but so jealous was he
of his secret, that he caused all his tools to be burnt before his
death;[320] nor was anything known of the processes by which Herschel
had achieved his astonishing results. Moreover, Lord Oxmantown had no
skilled workmen to assist him. His implements, both animate and
inanimate, had to be formed by himself. Peasants taken from the plough
were educated by him into efficient mechanics and engineers. The
delicate and complex machinery needed in operations of such hairbreadth
nicety as his enterprise involved, the steam-engine which was to set it
in motion, at times the very crucibles in which his specula were cast,
issued from his own workshops.
In 1827 experiments on the composition of speculum-metal were set on
foot, and the first polishing-machine ever driven by steam-power was
contrived in 1828. But twelve arduous years of struggle with recurring
difficulties passed before success began to dawn. A material less
tractable than the alloy selected, of four chemical equivalents of
copper to one of tin,[321] can scarcely be conceived. It is harder than
steel, yet brittle as glass, crumbling into fragments with the slightest
inadvertence of handling or treatment;[322] and the precision of figure
requisite to secure good definition is almost beyond the power of
language to convey. The quantities involved are so small as not alone to
elude sight, but to confound imagination. Sir John Herschel tells us
that "the _total_ thickness to be abraded from the edge of a spherical
speculum 48 inches in diameter and 40 feet focus, to convert it into a
paraboloid, is only 1/21333 of an inch;"[323] yet upon this minute
difference of form depends the clearness of the image, and, as a
consequence, the entire efficiency of the instrument. "Almost infinite,"
indeed (in the phrase of the late Dr. Robinson), must be the exactitude
of the operation adapted to bring about so delicate a result.
At length, in 1839, two specula, each three feet in diameter, were
turned out in such perfection as to prompt a still bolder experiment.
The various processes needed to insure success were now ascertained and
under control; all that was necessary was to repeat them on a larger
scale. A gigantic mirror, six feet across and fifty-four in focal
length, was accordingly cast on the 13th of April, 1842; in two months
it was ground down to figure by abrasion with emery and water, and
daintily polished with rouge; and by the month of February, 1845, the
"leviathan of Parsonstown" was available for the examination of the
heavens.
The suitable mounting of this vast machine was a problem scarcely less
difficult than its construction. The shape of a speculum needs to be
maintained with an elaborate care equal to that used in imparting it. In
fact, one of the most formidable obstacles to increasing the size of
such reflecting surfaces consists in their liability to bend under their
own weight. That of the great Rosse speculum was no less than four tons.
Yet, although six inches in thickness, and composed of a material only a
degree inferior in rigidity to wrought iron, the strong pressure of a
man's hand at its back produced sufficient flexure to distort
perceptibly the image of a star reflected in it.[324] Thus the delicacy
of its form was perishable equally by the stress of its own gravity, and
by the slightest irregularity in the means taken to counteract that
stress. The problem of affording a perfectly equable support in all
possible positions was solved by resting the speculum upon twenty-seven
platforms of cast iron, felt-covered, and carefully fitted to the shape
of the areas they were to carry, which platforms were themselves borne
by a complex system of triangles and levers, ingeniously adapted to
distribute the weight with complete uniformity.[325]
A tube which resembled, when erect, one of the ancient round towers of
Ireland,[326] served as the habitation of the great mirror. It was
constructed of deal staves bound together with iron hoops, was
fifty-eight feet long (including the speculum-box), and seven in
diameter. A reasonably tall man may walk through it (as Dean Peacock
once did) with umbrella uplifted. Two piers of solid masonry, about
fifty feet high, seventy long, and twenty-three apart, flanked the huge
engine on either side. Its lower extremity rested on a universal joint
of cast iron; above, it was slung in chains, and even in a gale of wind
remained perfectly steady. The weight of the entire, although amounting
to fifteen tons, was so skilfully counterpoised, that the tube could
with ease be raised or depressed by two men working a windlass. Its
horizontal range was limited by the lofty walls erected for its support
to about ten degrees on each side of the meridian; but it moved
vertically from near the horizon through the zenith as far as the pole.
Its construction was of the Newtonian kind, the observer looking into
the side of the tube near its upper end, which a series of galleries and
sliding stages enabled him to reach in any position. It has also, though
rarely, been used without a second mirror, as a "Herschelian" reflector.
The splendour of the celestial objects as viewed with this vast
"light-grasper" surpassed all expectation. "Never in my life," exclaimed
Sir James South, "did I see such glorious sidereal pictures."[327] The
orb of Jupiter produced an effect compared to that of the introduction
of a coach-lamp into the telescope;[328] and certain star-clusters
exhibited an appearance (we again quote Sir James South) "such as man
before had never seen, and which for its magnificence baffles all
description." But it was in the examination of the nebulæ that the
superiority of the new instrument was most strikingly displayed. A large
number of these misty objects, which the utmost powers of Herschel's
specula had failed to resolve into stars, yielded at once to the
Parsonstown reflector; while many others showed under entirely changed
forms through the disclosure of previously unseen details of structure.
One extremely curious result of the increase of light was the abolition
of any sharp distinction between the two classes of "annular" and
"planetary" nebulæ. Up to that time, only four ring-shaped systems--two
in the northern and two in the southern hemisphere--were known to
astronomers; they were now reinforced by five of the planetary kind, the
discs of which were observed to be centrally perforated; while the
definite margins visible in weaker instruments were replaced by ragged
edges or filamentous fringes.
Still more striking was the discovery of an entirely new and most
remarkable species of nebulæ. These were termed "spiral," from the more
or less regular convolutions, resembling the whorls of a shell, in which
the matter composing them appeared to be distributed. The first and most
conspicuous specimen of this class was met with in April, 1845; it is
situated in Canes Venatici, close to the tail of the Great Bear, and
wore, in Sir J. Herschel's instruments, the aspect of a split ring
encompassing a bright nucleus, thus presenting, as he supposed, a
complete analogue to the system of the Milky Way. In the Rosse mirror it
shone out as a vast whirlpool of light--a stupendous witness to the
presence of cosmical activities on the grandest scale, yet regulated by
laws as to the nature of which we are profoundly ignorant. Professor
Stephen Alexander of New Jersey, however, concluded, from an
investigation (necessarily founded on highly precarious data) of the
mechanical condition of these extraordinary agglomerations, that we see
in them "the partially scattered fragments of enormous masses once
rotating in a state of dynamical equilibrium." He further suggested
"that the separation of these fragments may still be in progress,"[329]
and traced back their origin to the disruption, through its own
continually accelerated rotation, of a "primitive spheroid" of
inconceivably vast dimensions. Such also, it was added (the curvilinear
form of certain outliers of the Milky Way giving evidence of a spiral
structure), is probably the history of our own cluster; the stars
composing which, no longer held together in a delicately adjusted system
like that of the sun and planets, are advancing through a period of
seeming confusion towards an appointed goal of higher order and more
perfect and harmonious adaptation.[330]
The class of spiral nebulæ included, in 1850, fourteen members, besides
several in which the characteristic arrangement seemed partial or
dubious.[331] A tendency in the exterior stars of other clusters to
gather into curved branches (as in our Galaxy) was likewise noted; and
the existence of unsuspected analogies was proclaimed by the significant
combination in the "Owl" nebula (a large planetary in Ursa Major)[332]
of the twisted forms of a spiral with the perforated effect distinctive
of an annular nebula.
Once more, by the achievements of the Parsonstown reflector, the
supposition of a "shining fluid" filling vast regions of space was
brought into (as it has since proved) undeserved discredit. Although
Lord Rosse himself rejected the inference, that because many nebulæ had
been resolved, all were resolvable, very few imitated his truly
scientific caution; and the results of Bond's investigations[333] with
the Harvard College refractor quickened and strengthened the current of
prevalent opinion. It is now certain that the evidence furnished on both
sides of the Atlantic as to the stellar composition of some conspicuous
objects of this class (notably the Orion and "Dumb-bell" nebulæ) was
delusive; but the spectroscope alone was capable of meeting it with a
categorical denial. Meanwhile there seemed good ground for the
persuasion, which now, for the last time, gained the upper hand, that
nebulæ are, without exception, true "island-universes," or assemblages
of distant suns.
Lord Rosse's telescope possesses a nominal power of 6,000--that is, it
shows the moon as if viewed with the naked eye at a distance of forty
miles. But this seeming advantage is neutralised by the weakening of the
available light through excessive diffusion, as well as by the troubles
of the surging sea of air through which the observation must necessarily
be made. Professor Newcomb, in fact, doubts whether with _any_ telescope
our satellite has ever been seen to such advantage as it would be if
brought within 500 miles of the unarmed eye.[334]
The French opticians' rule of doubling the number of millimetres
contained in the aperture of an instrument to find the highest
magnifying power usually applicable to it, would give 3,600 as the
maximum for the leviathan of Birr Castle; but in a climate like that of
Ireland the occasions must be rare when even that limit can be reached.
Indeed, the experience acquired by its use plainly shows that
atmospheric rather than mechanical difficulties impede a still further
increase of telescopic power. Its construction may accordingly be said
to mark the _ne plus ultra_ of effort in one direction, and the
beginning of its conversion towards another. It became thenceforward
more and more obvious that the conditions of observation must be
ameliorated before any added efficacy could be given to it. The full
effect of an uncertain climate in nullifying optical improvements was
recognised, and the attention of astronomers began to be turned towards
the advantages offered by more tranquil and more translucent skies.
Scarcely less important for the practical uses of astronomy than the
optical qualities of the telescope is the manner of its mounting. The
most admirable performance of the optician can render but unsatisfactory
service if its mechanical accessories are ill-arranged or inconvenient.
Thus the astronomer is ultimately dependent upon the mechanician; and so
excellently have his needs been served, that the history of the
ingenious contrivances by which discoveries have been prepared would
supply a subject (here barely glanced at) not far inferior in extent and
instruction to the history of those discoveries themselves.
There are two chief modes of using the telescope, to which all others
may be considered subordinate.[335] Either it may be invariably directed
towards the south, with no motion save in the plane of the meridian, so
as to intercept the heavenly bodies at the moment of transit across that
plain; or it may be arranged so as to follow the daily revolution of the
sky, thus keeping the object viewed permanently in sight instead of
simply noting the instant of its flitting across the telescopic field.
The first plan is that of the "transit instrument," the second that of
the "equatoreal." Both were, by a remarkable coincidence, introduced
about 1690[336] by Olaus Römer, the brilliant Danish astronomer who
first measured the velocity of light.
The uses of each are entirely different. With the transit, the really
fundamental task of astronomy--the determination of the movements of the
heavenly bodies--is mainly accomplished; while the investigation of
their nature and peculiarities is best conducted with the equatoreal.
One is the instrument of mathematical, the other of descriptive
astronomy. One furnishes the materials with which theories are
constructed and the tests by which they are corrected; the other
registers new facts, takes note of new appearances, sounds the depths
and peers into every nook of the heavens.
The great improvement of giving to a telescope equatoreally mounted an
automatic movement by connecting it with clockwork, was proposed in 1674
by Robert Hooke. Bradley in 1721 actually observed Mars with a telescope
"moved by a machine that made it keep pace with the stars;"[337] and Von
Zach relates[338] that he had once followed Sirius for twelve hours with
a "heliostat" of Ramsden's construction. But these eighteenth-century
attempts were of no practical effect. Movement by clockwork was
virtually a complete novelty when it was adopted by Fraunhofer in 1824
to the Dorpat refractor. By simply giving to an axis unvaryingly
directed towards the celestial pole an equable rotation with a period of
twenty-four hours, a telescope attached to it, and pointed in _any_
direction, will trace out on the sky a parallel of declination, thus
necessarily accompanying the movement of any star upon which it may be
fixed. It accordingly forms part of the large sum of Fraunhofer's merits
to have secured this inestimable advantage to observers.
Sir John Herschel considered that Lassell's application of equatoreal
mounting to a nine-inch Newtonian in 1840 made an epoch in the history
of "that eminently British instrument, the reflecting telescope."[339]
Nearly a century earlier,[340] it is true, Short had fitted one of his
Gregorians to a complicated system of circles in such a manner that, by
moving a handle, it could be made to follow the revolution of the sky;
but the arrangement did not obtain, nor did it deserve, general
adoption. Lassell's plan was a totally different one; he employed the
crossed axes of the true equatoreal, and his success removed, to a great
extent, the fatal objection of inconvenience in use, until then
unanswerably urged against reflectors. The very largest of these can now
be mounted equatoreally; even the Rosse, within its limited range, has
been for some years provided with a movement by clockwork along
declination-parallels.
The art of accurately dividing circular arcs into the minute equal parts
which serve as the units of astronomical measurement, remained, during
the whole of the eighteenth century, almost exclusively in English
hands. It was brought to a high degree of perfection by Graham, Bird and
Ramsden, all of whom, however, gave the preference to the old-fashioned
mural quadrant and zenith-sector over the entire circle, which Römer had
already found the advantage of employing. The five-foot vertical circle,
which Piazzi with some difficulty induced Ramsden to complete for him in
1789, was the first divided instrument constructed in what may be called
the modern style. It was provided with magnifiers for reading off the
divisions (one of the neglected improvements of Römer), and was set up
above a smaller horizontal circle, forming an "altitude and azimuth"
combination (again Römer's invention), by which both the elevation of a
celestial object above the horizon and its position as referred to the
horizon could be measured. In the same year, Borda invented the
"repeating circle" (the principle of which had been suggested by Tobias
Mayer in 1756[341]), a device for exterminating, so far as possible,
errors of graduation by _repeating_ an observation with different parts
of the limb. This was perhaps the earliest systematic effort to correct
the imperfections of instruments by the manner of their use.
The manufacture of astronomical circles was brought to a very refined
state of excellence early in the nineteenth century by Reichenbach at
Munich, and after 1818 by Repsold at Hamburg. Bessel states[342] that
the "reading-off" on an instrument of the kind by the latter artist was
accurate to about 1/80th of a human hair. Meanwhile the traditional
reputation of the English school was fully sustained; and Sir George
Airy did not hesitate to express his opinion that the new method of
graduating circles, published by Troughton in 1809,[343] was the
"greatest improvement ever made in the art of instrument-making."[344]
But a more secure road to improvement than that of mere mechanical
exactness was pointed out by Bessel. His introduction of a regular
theory of instrumental errors might almost be said to have created a new
art of observation. Every instrument, he declared in memorable
words,[345] must be twice made--once by the artist, and again by the
observer. Knowledge is power. Defects that are ascertained and can be
allowed for are as good as non-existent. Thus the truism that the best
instrument is worthless in the hands of a careless or clumsy observer,
became supplemented by the converse maxim, that defective appliances
may, through skilful use, be made to yield valuable results. The
Königsberg observations--of which the first instalment was published in
1815--set the example of regular "reduction" for instrumental errors.
Since then, it has become an elementary part of an astronomer's duty to
study the _idiosyncrasy_ of each one of the mechanical contrivances at
his disposal, in order that its inevitable, but now certified deviations
from ideal accuracy may be included amongst the numerous corrections by
which the pure essence of even approximate truth is distilled from the
rude impressions of sense.
Nor is this enough; for the casual circumstances attending each
observation have to be taken into account with no less care than the
inherent or _constitutional_ peculiarities of the instrument with which
it is made. There is no "once for all" in astronomy. Vigilance can never
sleep; patience can never tire. Variable as well as constant sources of
error must be anxiously heeded; one infinitesimal inaccuracy must be
weighed against another; all the forces and vicissitudes of
nature--frosts, dews, winds, the interchanges of heat, the disturbing
effects of gravity, the shiverings of the air, the tremors of the earth,
the weight and vital warmth of the observer's own body, nay, the rate at
which his brain receives and transmits its impressions, must all enter
into his calculations, and be sifted out from his results.
It was in 1823 that Bessel drew attention to discrepancies in the times
of transits given by different astronomers.[346] The quantities involved
were far from insignificant. He was himself nearly a second in advance
of all his contemporaries, Argelander lagging behind him as much as a
second and a quarter. Each individual, in fact, was found to have a
certain definite _rate of perception_, which, under the name of
"personal equation," now forms so important an element in the correction
of observations that a special instrument for accurately determining its
amount in each case is in actual use at Greenwich.
Such are the refinements upon which modern astronomy depends for its
progress. It is a science of hairbreadths and fractions of a second. It
exists only by the rigid enforcement of arduous accuracy and unwearying
diligence. Whatever secrets the universe still has in store for man will
only be communicated on these terms. They are, it must be acknowledged,
difficult to comply with. They involve an unceasing struggle against the
infirmities of his nature and the instabilities of his position. But the
end is not unworthy the sacrifices demanded. One additional ray of light
thrown on the marvels of creation--a single, minutest encroachment upon
the strongholds of ignorance--is recompense enough for a lifetime of
toil. Or rather, the toil is its own reward, if pursued in the lofty
spirit which alone becomes it. For it leads through the abysses of space
and the unending vistas of time to the very threshold of that infinity
and eternity of which the disclosure is reserved for a life to come.
FOOTNOTES:
[Footnote 305: Grant, _Hist. Astr._, p. 527.]
[Footnote 306: _Optica Promota_, p. 93.]
[Footnote 307: _Phil. Trans._, vol. xxxii., p. 383.]
[Footnote 308: _Ibid._, vol. xc., p. 65.]
[Footnote 309: Cassegrain, a Frenchman, substituted in 1672 a _convex_
for a _concave_ secondary speculum. The tube was thereby enabled to be
shortened by twice the focal length of the mirror in question. The great
Melbourne reflector (four feet aperture, by Grubb) is constructed upon
this plan.]
[Footnote 310: _Phil. Trans._, vol. civ., p. 275, _note_.]
[Footnote 311: _Phil. Trans._, vol. xc., p. 70. With the forty-foot,
however, only very moderate powers seemed to have been employed, whence
Dr. Robinson argued a deficiency of defining power. _Proc. Roy. Irish
Ac._, vol. ii., p. 11.]
[Footnote 312: _Phil. Trans._, vol. lxxi., p. 492.]
[Footnote 313: It is remarkable that, as early as 1695, the possibility
of an achromatic combination was inferred by David Gregory from the
structure of the human eye. See his _Catoptricæ et Dioptricæ Sphericæ
Elementa_, p. 98.]
[Footnote 314: Wolf, _Biographien_, Bd. ii., p. 301.]
[Footnote 315: _Month. Not._, vol. i., p. 153. _note_.]
[Footnote 316: Henrivaux, _Encyclopédie Chimique_, t. v., fasc. 5, p.
363.]
[Footnote 317: See _ante_, p. 83.]
[Footnote 318: _Phil. Trans._, vol. vii., p. 4007.]
[Footnote 319: J. Herschel, _The Telescope_, p. 39.]
[Footnote 320: _Month. Not._, vol. xxix., p. 125.]
[Footnote 321: A slight excess of copper renders the metal easier to
work, but liable to tarnish. Robinson, _Proc. Roy. Irish Ac._, vol. ii.,
p. 4.]
[Footnote 322: _Brit. Ass._, 1843, Dr. Robinson's closing Address.
_Athenæum_, Sept. 23, p. 866.]
[Footnote 323: _The Telescope_, p. 82.]
[Footnote 324: Lord Rosse in _Phil. Trans._, vol. cxl., p. 302.]
[Footnote 325: This method is the same in principle with that applied by
Grubb in 1834 to a 15-inch speculum for the observatory of Armagh.
_Phil. Trans._, vol. clix., p. 145.]
[Footnote 326: Robinson, _Proc. Roy. Ir. Ac._, vol. iii., p. 120.]
[Footnote 327: _Astr. Nach._, No. 536.]
[Footnote 328: Airy, _Month. Not._, vol. ix., p. 120.]
[Footnote 329: _Astronomical Journal_ (Gould's), vol. ii., p. 97.]
[Footnote 330: _Ibid._, p. 160.]
[Footnote 331: Lord Rosse in _Phil. Trans._, vol. cxl., p. 505.]
[Footnote 332: No. 2343 of Herschel's (1864) Catalogue. Before 1850 a
star was visible in each of the two larger openings by which it is
pierced; since then, one only. Webb, _Celestial Objects_ (4th ed.), p.
409.]
[Footnote 333: _Mem. Am. Ac._, vol. iii., p. 87; _Astr. Nach._, No.
611.]
[Footnote 334: _Pop. Astr._, p. 145.]
[Footnote 335: This statement must be taken in the most general sense.
Supplementary observations of great value are now made at Greenwich with
the altitude and azimuth instrument, which likewise served Piazzi to
determine the places of his stars; while a "prime vertical instrument"
is prominent at Pulkowa.]
[Footnote 336: As early as 1620, according to R. Wolf (_Ges. der Astr._,
p. 587), Father Scheiner made the experiment of connecting a telescope
with an axis directed to the pole, while Chinese "equatoreal armillæ,"
dating from the thirteenth century, existed at Pekin until 1900, when
they were carried off as "loot" to Berlin. J. L. E. Dreyer,
_Copernicus_, vol. i., p. 134.]
[Footnote 337: _Miscellaneous Works_, p. 350.]
[Footnote 338: _Astr. Jahrbuch_, 1799 (published 1796), p. 115.]
[Footnote 339: _Month. Not._, vol. xli., p. 189.]
[Footnote 340: _Phil. Trans._, vol. xlvi., p. 242.]
[Footnote 341: Grant, _Hist. of Astr._, p. 487.]
[Footnote 342: _Pop. Vorl._, p. 546.]
[Footnote 343: _Phil. Trans._, vol. xcix., p. 105.]
[Footnote 344: _Report Brit. Ass._, 1832, p. 132.]
[Footnote 345: _Pop. Vorl._, p. 432.]
[Footnote 346: C. T. Anger, _Grundzüge der neucren astronomischen
Beobachtungs-Kunst_, p. 3.]
PART II
RECENT PROGRESS OF ASTRONOMY
CHAPTER I
_FOUNDATION OF ASTRONOMICAL PHYSICS_
In the year 1826, Heinrich Schwabe of Dessau, elated with the hope of
speedily delivering himself from the hereditary incubus of an
apothecary's shop,[347] obtained from Munich a small telescope and began
to observe the sun. His choice of an object for his researches was
instigated by his friend Harding of Göttingen. It was a peculiarly happy
one. The changes visible in the solar surface were then generally
regarded as no less capricious than the changes in the skies of our
temperate regions. Consequently, the reckoning and registering of
sun-spots was a task hardly more inviting to an astronomer than the
reckoning and registering of summer clouds. Cassini, Keill, Lemonnier,
Lalande, were unanimous in declaring that no trace of regularity could
be detected in their appearances or effacements.[348] Delambre
pronounced them "more curious than really useful."[349] Even Herschel,
profoundly as he studied them, and intimately as he was convinced of
their importance as symptoms of solar activity, saw no reason to suspect
that their abundance and scarcity were subject to orderly alternation.
One man alone in the eighteenth century, Christian Horrebow of
Copenhagen, divined their periodical character, and foresaw the time
when the effects of the sun's vicissitudes upon the globes revolving
round him might be investigated with success; but this prophetic
utterance was of the nature of a soliloquy rather than of a
communication, and remained hidden away in an unpublished journal until
1859, when it was brought to light in a general ransacking of
archives.[350]
Indeed, Schwabe himself was far from anticipating the discovery which
fell to his share. He compared his fortune to that of Saul, who, seeking
his father's asses, found a kingdom.[351] For the hope which inspired
his early resolution lay in quite another direction. His patient ambush
was laid for a possible intramercurial planet, which, he thought, must
sooner or later betray its existence in crossing the face of the sun. He
took, however, the most effectual measures to secure whatever new
knowledge might be accessible. During forty-three years his
"imperturbable telescope"[352] never failed, weather and health
permitting, to bring in its daily report as to how many, or if any,
spots were visible on the sun's disc, the information obtained being day
by day recorded on a simple and unvarying system. In 1843 he made his
first announcement of a probable decennial period,[353] but it met with
no general attention; although Julius Schmidt of Bonn (afterwards
director of the Athens Observatory) and Gautier of Geneva were impressed
with his figures, and Littrow had himself, in 1836,[354] hinted at the
likelihood of some kind of regular recurrence. Schwabe, however, worked
on, gathering each year fresh evidence of a law such as he had
indicated; and when Humboldt published in 1851, in the third volume of
his _Kosmos_,[355] a table of the sun-spot statistics collected by him
from 1826 downwards, the strength of his case was perceived with, so to
speak, a start of surprise; the reality and importance of the discovery
were simultaneously recognised, and the persevering Hofrath of Dessau
found himself famous among astronomers. His merit--recognised by the
bestowal of the Astronomical Society's Gold Medal in 1857--consisted in
his choice of an original and appropriate line of work, and in the
admirable tenacity of purpose with which he pursued it. His resources
and acquirements were those of an ordinary amateur; he was distinguished
solely by the unfortunately rare power of turning both to the best
account. He died where he was born and had lived, April 11, 1875, at the
ripe age of eighty-six.
Meanwhile an investigation of a totally different character, and
conducted by totally different means, had been prosecuted to a very
similar conclusion. Two years after Schwabe began his solitary
observations, Humboldt gave the first impulse, at the Scientific
Congress of Berlin in 1828, to a great international movement for
attacking simultaneously, in various parts of the globe, the complex
problem of terrestrial magnetism. Through the genius and energy of
Gauss, Göttingen became its centre. Thence new apparatus, and a new
system for its employment, issued; there, in 1833, the first regular
magnetic observatory was founded, whilst at Göttingen was fixed the
universal time-standard for magnetic observations. A letter addressed by
Humboldt in April, 1836, to the Duke of Sussex as President of the Royal
Society, enlisted the co-operation of England. A network of magnetic
stations was spread all over the British dominions, from Canada to Van
Diemen's Land; measures were concerted with foreign authorities, and an
expedition was fitted out, under the able command of Captain (afterwards
Sir James) Clark Ross, for the special purpose of bringing intelligence
on the subject from the dismal neighbourhood of the South Pole. In 1841,
the elaborate organisation created by the disinterested efforts of
scientific "agitators" was complete; Gauss's "magnetometers" were
vibrating under the view of attentive observers in five continents, and
simultaneous results began to be recorded.
Ten years later, in September, 1851, Dr. John Lamont, the Scotch
director of the Munich Observatory, in reviewing the magnetic
observations made at Göttingen and Munich from 1835 to 1850, perceived
with some surprise that they gave unmistakable indications of a period
which he estimated at 10-1/3 years.[356] The manner in which this
periodicity manifested itself requires a word of explanation. The
observations in question referred to what is called the "declination" of
the magnetic needle--that is, to the position assumed by it with
reference to the points of the compass when moving freely in a
horizontal plane. Now this position--as was discovered by Graham in
1722--is subject to a small daily fluctuation, attaining its maximum
towards the east about 8 A.M., and its maximum towards the west shortly
before 2 P.M. In other words, the direction of the needle approaches (in
these countries at the present time) nearest to the true north some four
hours before noon, and departs farthest from it between one and two
hours after noon. It was the _range_ of this daily variation that Lamont
found to increase and diminish once in every 10-1/3 years.
In the following winter, Sir Edward Sabine, ignorant as yet of Lamont's
conclusion, undertook to examine a totally different set of
observations. The materials in his hands had been collected at the
British colonial stations of Toronto and Hobarton from 1843 to 1848, and
had reference, not to the regular diurnal swing of the needle, but to
those curious spasmodic vibrations, the inquiry into the laws of which
was the primary object of the vast organisation set on foot by Humboldt
and Gauss. Yet the upshot was practically the same. Once in about ten
years, magnetic disturbances (termed by Humboldt "storms") were
perceived to reach a maximum of violence and frequency. Sabine was the
first to note the coincidence between this unlooked-for result and
Schwabe's sun-spot period. He showed that, so far as observation had yet
gone, the two cycles of change agreed perfectly both in duration and
phase, maximum corresponding to maximum, minimum to minimum. What the
nature of the connection could be that bound together by a common law
effects so dissimilar as the rents in the luminous garment of the sun,
and the swayings to and fro of the magnetic needle, was and still
remains beyond the reach of well-founded theory; but the fact was from
the first undeniable.
The memoir containing this remarkable disclosure was presented to the
Royal Society, March 18, and read May 6, 1852.[357] On the 31st of July
following, Rudolf Wolf at Berne,[358] and on the 18th of August, Alfred
Gautier at Sion,[359] announced, separately and independently, perfectly
similar conclusions. This triple event is perhaps the most striking
instance of the successful employment of the Baconian method of
co-operation in discovery, by which "particulars" are amassed by one set
of investigators--corresponding to the "Depredators" and "Inoculators"
of Solomon's House--while inductions are drawn from them by another and
a higher class--the "Interpreters of Nature." Yet even here the
convergence of two distinct lines of research was wholly fortuitous, and
skilful combination owed the most brilliant part of its success to the
unsought bounty of what we call Fortune.
The exactness of the coincidence thus brought to light was fully
confirmed by further inquiries. A diligent search through the scattered
records of sun-spot observations, from the time of Galileo and Scheiner
onwards, put Wolf[360] in possession of materials by which he was
enabled to correct Schwabe's loosely-indicated decennial period to one
of slightly over eleven (11.11) years; and he further showed that this
fell in with the ebb and flow of magnetic change even better than
Lamont's 10-1/3 year cycle. The analogy was also pointed out between the
"light-curve," or zig-zagged line representing on paper the varying
intensity in the lustre of certain stars, and the similar delineation of
spot-frequency; the ascent from minimum to maximum being, in both cases,
usually steeper than the descent from maximum to minimum; while an
additional point of resemblance was furnished by the irregularities in
height of the various maxima. In other words, both the number of spots
on the sun and the brightness of variable stars increase, as a rule,
more rapidly than they decrease; nor does the amount of that increase,
in either instance, show any approach to uniformity.
The endeavour, suggested by the very nature of the phenomenon, to
connect sun-spots with weather was less successful. The first attempt of
the kind was made by Sir William Herschel in 1801, and a very notable
one it was. Meteorological statistics, save of the scantiest and most
casual kind, did not then exist; but the price of corn from year to year
was on record, and this, with full recognition of its inadequacy, he
adopted as his criterion. Nor was he much better off for information
respecting the solar condition. What little he could obtain, however,
served, as he believed, to confirm his surmise that a copious emission
of light and heat accompanies an abundant formation of "openings" in the
dazzling substance whence our supply of those indispensable commodities
is derived.[361] He gathered, in short, from his inquiries very much
what he had expected to gather, namely, that the price of wheat was high
when the sun showed an unsullied surface, and that food and spots became
plentiful together.[362]
Yet this plausible inference was scarcely borne out by a more exact
collocation of facts. Schwabe failed to detect any reflection of the
sun-spot period in his meteorological register. Gautier[363] reached a
provisional conclusion the reverse--though not markedly the reverse--of
Herschel's. Wolf, in 1852, derived from an examination of Vogel's
collection of Zürich Chronicles (1000-1800 A.D.) evidence showing (as he
thought) that minimum years were usually wet and stormy, maximum years
dry and genial;[364] but a subsequent review of the subject in 1859
convinced him that no relation of any kind between the two kinds of
effects was traceable.[365] With the singular affection of our
atmosphere known as the Aurora Borealis (more properly Aurora Polaris)
the case was different. Here the Zürich Chronicles set Wolf on the right
track in leading him to associate such luminous manifestations with a
disturbed condition of the sun; since subsequent detailed observation
has exhibited the curve of auroral frequency as following with such
fidelity the jagged lines figuring to the eye the fluctuations of solar
and magnetic activity, as to leave no reasonable doubt that all three
rise and sink together under the influence of a common cause. As long
ago as 1716,[366] Halley had conjectured that the Northern Lights were
due to magnetic "effluvia," but there was no evidence on the subject
forthcoming until Hiorter observed at Upsala in 1741 their agitating
influence upon the magnetic needle. That the effect was no casual one
was made superabundantly clear by Arago's researches in 1819 and
subsequent years. Now both were perceived to be swayed by the same
obscure power of cosmical disturbance.
The sun is not the only one of the heavenly bodies by which the
magnetism of the earth is affected. Proofs of a similar kind of lunar
action were laid by Kreil in 1841 before the Bohemian Society of
Sciences, and with minor corrections were fully substantiated by
Sabine's more extended researches. It was thus ascertained that each
lunar day, or the interval of twenty-four hours and about fifty-four
minutes between two successive meridian passages of our satellite, is
marked by a perceptible, though very small, double oscillation of the
needle--two progressive movements from east to west, and two returns
from west to east.[367] Moreover, the lunar, like the solar influence
(as was proved in each case by Sabine's analysis of the Hobarton and
Toronto observations), extends to all three "magnetic elements,"
affecting not only the position of the horizontal or _declination_
needle, but also the dip and intensity. It seems not unreasonable to
attribute some portion of the same subtle power to the planets and even
to the stars, though with effects rendered imperceptible by distance.
We have now to speak of the discovery and application to the heavenly
bodies of a totally new method of investigation. Spectrum analysis may
be shortly described as a mode of distinguishing the various species of
matter by the kind of light proceeding from each. This definition at
once explains how it is that, unlike every other system of chemical
analysis, it has proved available in astronomy. Light, so far as
_quality_ is concerned, ignores distance. No intrinsic change, that we
yet know of, is produced in it by a journey from the farthest bounds of
the visible universe; so that, provided only that in _quantity_ it
remain sufficient for the purpose, its peculiarities can be equally well
studied whether the source of its vibrations be one foot or a hundred
billion miles distant. Now the most obvious distinction between one kind
of light and another resides in colour. But of this distinction the eye
takes cognisance in an æsthetic, not in a scientific sense. It finds
gladness in the "thousand tints" of nature, but can neither analyse nor
define them. Here the refracting prism--or the combination of prisms
known as the "spectroscope"--comes to its aid, teaching it to measure as
well as to perceive. It furnishes, in a word, an accurate scale of
colour. The various rays which, entering the eye together in a confused
crowd, produce a compound impression made up of undistinguishable
elements, are, by the mere passage through a triangular piece of glass,
separated one from the other, and ranged side by side in orderly
succession, so that it becomes possible to tell at a glance what kinds
of light are present, and what absent. Thus, if we could only be assured
that the various chemical substances when made to glow by heat, emit
characteristic rays--rays, that is, occupying a place in the spectrum
reserved for them, and for them _only_--we should at once be in
possession of a mode of identifying such substances with the utmost
readiness and certainty. This assurance, which forms the solid basis of
spectrum analysis, was obtained slowly and with difficulty.
The first to employ the prism in the examination of various flames (for
it is only in a state of vapour that matter emits distinctive light) was
a young Scotchman named Thomas Melvill, who died in 1753, at the age of
twenty-seven. He studied the spectrum of burning spirits, into which
were successively introduced sal ammoniac, potash, alum, nitre, and
sea-salt, and observed the singular predominance, under almost all
circumstances, of a particular shade of yellow light, perfectly definite
in its degree of refrangibility[368]--in other words, taking up a
perfectly definite position in the spectrum. His experiments were
repeated by Morgan,[369] Wollaston, and--with far superior precision and
diligence--by Fraunhofer.[370] The great Munich optician, whose work was
completely original, rediscovered Melvill's deep yellow ray and measured
its place in the colour-scale. It has since become well known as the
"sodium line," and has played a very important part in the history of
spectrum analysis. Nevertheless, its ubiquity and conspicuousness long
impeded progress. It was elicited by the combustion of a surprising
variety of substances--sulphur, alcohol, ivory, wood, paper; its
persistent visibility suggesting the accomplishment of some universal
process of nature rather than the presence of one individual kind of
matter. But if spectrum analysis were to exist as a science at all, it
could only be by attaining certainty as to the unvarying association of
one special substance with each special quality of light.
Thus perplexed, Fox Talbot[371] hesitated in 1826 to enounce this
fundamental principle. He was inclined to believe that the presence in
the spectrum of any individual ray told unerringly of the volatilisation
in the flame under scrutiny of some body as whose badge or distinctive
symbol that ray might be regarded; but the continual prominence of the
yellow beam staggered him. It appeared, indeed, without fail where
sodium _was_; but it also appeared where it might be thought only
reasonable to conclude that sodium _was not_. Nor was it until thirty
years later that William Swan,[372] by pointing out the extreme delicacy
of the spectral test, and the singularly wide dispersion of sodium, made
it appear probable (but even then only probable) that the questionable
yellow line was really due invariably to that substance. Common salt
(chloride of sodium) is, in fact, the most diffusive of solids. It
floats in the air; it flows with water; every grain of dust has its
attendant particle; its absolute exclusion approaches the impossible.
And withal, the light that it gives in burning is so intense and
concentrated, that if a single grain be divided into 180 million parts,
and one alone of such inconceivably minute fragments be present in a
source of light, the spectroscope will show unmistakably its
characteristic beam.
Amongst the pioneers of knowledge in this direction were Sir John
Herschel[373]--who, however, applied himself to the subject in the
interests of optics, not of chemistry--W. A. Miller,[374] and
Wheatstone. The last especially made a notable advance when, in the
course of his studies on the "prismatic decomposition" of the electric
light, he reached the significant conclusion that the rays visible in
its spectrum were different for each kind of metal employed as
"electrodes."[375] Thus indications of a wider principle were to be
found in several quarters, but no positive certainty on any single point
was obtained, until, in 1859, Gustav Kirchhoff, professor of physics in
the University of Heidelberg, and his colleague, the eminent chemist
Robert Bunsen, took the matter in hand. By them the general question as
to the necessary and invariable connection of certain rays in the
spectrum with certain kinds of matter, was first resolutely confronted,
and first definitely answered. It was answered affirmatively--else there
could have been no science of spectrum analysis--as the result of
experiments more numerous, more stringent, and more precise than had
previously been undertaken.[376] And the assurance of their conclusion
was rendered doubly sure by the discovery, through the peculiarities of
their light alone, of two new metals, named from the blue and red rays
by which they were respectively distinguished, "cæsium," and
"rubidium."[377] Both were immediately afterwards actually obtained in
small quantities by evaporation of the Durckheim mineral waters.
The link connecting this important result with astronomy may now be
indicated. In the year 1802 it occurred to William Hyde Wollaston to
substitute for the round hole used by Newton and his successors for the
admittance of light to be examined with the prism, an elongated
"crevice" 1/20th of an inch in width. He thereupon perceived that the
spectrum, thus formed of light, as it were, _purified_ by the abolition
of overlapping images, was traversed by seven dark lines. These he took
to be natural boundaries of the various colours,[378] and satisfied with
this quasi-explanation, allowed the subject to drop. It was
independently taken up after twelve years by a man of higher genius. In
the course of experiments on light, directed towards the perfecting of
his achromatic lenses, Fraunhofer, by means of a slit and a telescope,
made the surprising discovery that the solar spectrum is crossed, not by
seven, but by thousands of obscure transverse streaks.[379] Of these he
counted some 600, and carefully mapped 324, while a few of the most
conspicuous he set up (if we may be permitted the expression) as
landmarks, measuring their distances apart with a theodolite, and
affixing to them the letters of the alphabet, by which they are still
universally known. Nor did he stop here. The same system of examination
applied to the rest of the heavenly bodies showed the mild effulgence of
the moon and planets to be deficient in precisely the same rays as
sunlight; while in the stars it disclosed the differences in likeness
which are always an earnest of increased knowledge. The spectra of
Sirius and Castor, instead of being delicately ruled crosswise
throughout, like that of the sun, were seen to be interrupted by three
massive bars of darkness--two in the blue and one in the green;[380] the
light of Pollux, on the other hand, seemed precisely similar to sunlight
attenuated by distance or reflection, and that of Capella, Betelgeux,
and Procyon to share some of its peculiarities. One solar line
especially--that marked in his map with the letter D--proved common to
all the four last-mentioned stars; and it was remarkable that it exactly
coincided in position with the conspicuous yellow beam (afterwards, as
we have said, identified with the light of glowing sodium) which he had
already found to accompany most kinds of combustion. Moreover, both the
_dark_ solar and the _bright_ terrestrial "D lines" were displayed by
the refined Munich appliances as double.
In this striking correspondence, discovered by Fraunhofer in 1815, was
contained the very essence of solar chemistry; but its true significance
did not become apparent until long afterwards. Fraunhofer was by
profession, not a physicist, but a practical optician. Time pressed; he
could not and would not deviate from his appointed track; all that was
possible to him was to indicate the road to discovery, and exhort others
to follow it.[381]
Partially and inconclusively at first this was done. The "fixed lines"
(as they were called) of the solar spectrum took up the position of a
standing problem, to the solution of which no approach seemed possible.
Conjectures as to their origin were indeed rife. An explanation put
forward by Zantedeschi[382] and others, and dubiously favoured by Sir
David Brewster and Dr. J. H. Gladstone,[383] was that they resulted from
"interference"--that is, a destruction of the motion producing in our
eyes the sensation of light, by the superposition of two light-waves in
such a manner that the crests of one exactly fill up the hollows of the
other. This effect was supposed to be brought about by imperfections in
the optical apparatus employed.
A more plausible view was that the atmosphere of the earth was the agent
by which sunlight was deprived of its missing beams. For a few of them
this is actually the case. Brewster found in 1832 that certain dark
lines, which were invisible when the sun stood high in the heavens,
became increasingly conspicuous as he approached the horizon.[384] These
are the well-known "atmospheric lines;" but the immense majority of
their companions in the spectrum remain quite unaffected by the
thickness of the stratum of air traversed by the sunlight containing
them. They are then obviously due to another cause.
There remained the true interpretation--absorption in the _sun's_
atmosphere; and this, too, was extensively canvassed. But a remarkable
observation made by Professor Forbes of Edinburgh[385] on the occasion
of the annular eclipse of May 15, 1836, appeared to throw discredit upon
it. If the problematical dark lines were really occasioned by the
stoppage of certain rays through the action of a vaporous envelope
surrounding the sun, they ought, it seemed, to be strongest in light
proceeding from his edges, which, cutting that envelope obliquely,
passed through a much greater depth of it. But the circle of light left
by the interposing moon, and of course derived entirely from the rim of
the solar disc, yielded to Forbes's examination precisely the same
spectrum as light coming from its central parts. This circumstance
helped to baffle inquirers, already sufficiently perplexed. It still
remains an anomaly, of which no satisfactory explanation has been
offered.
Convincing evidence as to the true nature of the solar lines was however
at length, in the autumn of 1859, brought forward at Heidelburg.
Kirchhoff's _experimentum crucis_ in the matter was a very simple one.
He threw bright sunshine across a space occupied by vapour of sodium,
and perceived with astonishment that the dark Fraunhofer line D, instead
of being effaced by flame giving a luminous ray of the same
refrangibility, was deepened and thickened by the superposition.
He tried the same experiment, substituting for sunbeams light from a
Drummond lamp, and with similar result. A dark furrow, corresponding in
every respect to the solar D-line, was instantly seen to interrupt the
otherwise unbroken radiance of its spectrum. The inference was
irresistible, that the effect thus produced artificially was brought
about naturally in the same way, and that sodium formed an ingredient in
the glowing atmosphere of the sun.[386] This first discovery was quickly
followed up by the identification of numerous bright rays in the spectra
of other metallic bodies with others of the hitherto mysterious
Fraunhofer lines. Kirchhoff was thus led to the conclusion that (besides
sodium) iron, magnesium, calcium, and chromium, are certainly solar
constituents, and that copper, zinc, barium, and nickel are also
present, though in smaller quantities.[387] As to cobalt, he hesitated
to pronounce, but its existence in the sun has since been established.
These memorable results were founded upon a general principle first
enunciated by Kirchhoff in a communication to the Berlin Academy,
December 15, 1859, and afterwards more fully developed by him.[388] It
may be expressed as follows: Substances of every kind are opaque to the
precise rays which they emit at the same temperature; that is to say,
they stop the kinds of light or heat which they are then actually in a
condition to radiate. But it does not follow that _cool_ bodies absorb
the rays which they would give out if sufficiently heated. Hydrogen at
ordinary temperatures, for instance, is almost perfectly transparent,
but if raised to the glowing point--as by the passage of electricity--it
_then_ becomes capable of arresting, and at the same time of displaying
in its own spectrum light of four distinct colours.
This principle is fundamental to solar chemistry. It gives the key to
the hieroglyphics of the Fraunhofer lines. The identical characters
which are written _bright_ in terrestrial spectra are written _dark_ in
the unrolled sheaf of sun-rays; the meaning remains unchanged. It must,
however, be remembered that they are only _relatively_ dark. The
substances stopping those particular tints in the neighbourhood of the
sun are at the same time vividly glowing with the very same. Remove the
dazzling solar background, by contrast with which they show as obscure,
and they will be seen, and, at critical moments, actually have been
seen, in all their native splendour. It is because the atmosphere of the
sun is cooler than the globe it envelops that the different kinds of
vapour constituting that atmosphere take more than they give, absorb
more light than they are capable of emitting; raise them to the same
temperature as the sun itself, and their powers of emission and
absorption being brought exactly to the same level, the thousands of
dusky rays in the solar spectrum will be at once obliterated.
The establishment of the terrestrial science of spectrum analysis was
due, as we have seen, equally to Kirchhoff and Bunsen, but its celestial
application to Kirchhoff alone. He effected this object of the
aspirations, more or less dim, of many other thinkers and workers, by
the union of two separate, though closely related lines of research--the
study of the different kinds of light _emitted_ by various bodies, and
the study of the different kinds of light _absorbed_ by them. The latter
branch appears to have been first entered upon by Dr. Thomas Young in
1803;[389] it was pursued by the younger Herschel,[390] by William Allen
Miller, Brewster, and Gladstone. Brewster indeed made, in 1833,[391] a
formal attempt to found what might be called an inverse system of
analysis with the prism based upon absorption; and his efforts were
repeated, just a quarter of a century later, by Gladstone.[392] But no
general point of view was attained; nor, it may be added, was it by this
path attainable.
Kirchhoff's map of the solar spectrum, drawn to scale with exquisite
accuracy, and printed in three shades of ink to convey the graduated
obscurity of the lines, was published in the Transactions of the Berlin
Academy for 1861 and 1862.[393] Representations of the principal lines
belonging to various elementary bodies formed, as it were, a series of
marginal notes accompanying the great solar scroll, enabling the veriest
tiro in the new science to decipher its meaning at a glance. Where the
dark solar and bright metallic rays agreed in position, it might safely
be inferred that the metal emitting them was a solar constituent; and
such coincidences were numerous. In the case of iron alone, no less than
sixty occurred in one-half of the spectral area, rendering the
chances[394] absolutely overwhelming against mere casual conjunction.
The preparation of this elaborate picture proved so trying to the eyes
that Kirchhoff was compelled by failing vision to resign the latter half
of the task to his pupil Hofmann. The complete map measured nearly eight
feet in length.
The conclusions reached by Kirchhoff were no sooner announced than they
took their place, with scarcely a dissenting voice, among the
established truths of science. The broad result, that the dark lines in
the spectrum of the sun afford an index to its chemical composition no
less reliable than any of the tests used in the laboratory, was equally
captivating to the imagination of the vulgar, and authentic in the
judgment of the learned; and, like all genuine advances in the knowledge
of Nature, it stimulated curiosity far more than it gratified it. Now
the history of how discoveries were missed is often quite as instructive
as the history of how they were made; it may then be worth while to
expend a few words on the thoughts and trials by which, in the present
case, the actual event was heralded.
Three times it seemed on the verge of being anticipated. The experiment,
which in Kirchhoff's hands proved decisive, of passing sunlight through
glowing vapours and examining the superposed spectra, was performed by
Professor W. A. Miller of King's College in 1845.[395] Nay, more, it was
performed with express reference to the question, then already (as has
been noted) in debate, of the possible production of Fraunhofer's lines
by absorption in a solar atmosphere. Yet it led to nothing.
Again, at Paris in 1849, with a view to testing the asserted coincidence
between the solar D-line and the bright yellow beam in the spectrum of
the electric arc (really due to the unsuspected presence of sodium),
Léon Foucault threw a ray of sunshine across the arc and observed its
spectrum.[396] He was surprised to see that the D-line was rendered more
intensely dark by the combination of lights. To assure himself still
further, he substituted a reflected image of one of the white-hot
carbon-points for the sunbeam, with an identical result. _The same ray
was missing._ It needed but another step to have generalised this
result, and thus laid hold of a natural truth of the highest importance;
but that step was not taken. Foucault, keen and brilliant though he was,
rested satisfied with the information that the _voltaic arc_ had the
power of stopping the kind of light emitted by it; he asked no further
question, and was consequently the bearer of no further intelligence on
the subject.
The truth conveyed by this remarkable experiment was, however, divined
by one eminent man. Professor Stokes of Cambridge stated to Sir William
Thomson (now Lord Kelvin), shortly after it had been made, his
conviction that an absorbing atmosphere of sodium surrounded the sun.
And so forcibly was his hearer impressed with the weight of the argument
based upon the absolute agreement of the D-line in the solar spectrum
with the yellow ray of burning sodium (then freshly certified by W. H.
Miller), combined with Foucault's "reversal" of that ray, that he
regularly inculcated, in his public lectures on natural philosophy at
Glasgow, five or six years before Kirchhoff's discovery, not only the
_fact_ of the presence of sodium in the solar neighbourhood, but also
the _principle_ of the study of solar and stellar chemistry in the
spectra of flames.[397] Yet it does not appear to have occurred to
either of these two distinguished professors--themselves among the
foremost of their time in the successful search for new truths--to
verify practically a sagacious conjecture in which was contained the
possibility of a scientific revolution. It is just to add, that
Kirchhoff was unacquainted, when he undertook his investigation, either
with the experiment of Foucault or the speculation of Stokes.
For C. J. Ångström, on the other hand, perhaps somewhat too much has
been claimed in the way of anticipation. His _Optical Researches_
appeared at Upsala in 1853, and in their English garb two years
later.[398] They were undoubtedly pregnant with suggestion, yet made no
epoch in discovery. The old perplexities continued to prevail after, as
before their publication. To Ångström, indeed, belongs the great merit
of having revived Euler's principle of the equivalence of emission and
absorption; but he revived it in its original crude form, and without
the qualifying proviso which alone gave it value as a clue to new
truths. According to his statement, a body absorbs all the series of
vibrations it is, under any circumstances, capable of emitting, as well
as those connected with them by simple harmonic relations. This is far
too wide. To render it either true or useful, it had to be reduced to
the cautious terms employed by Kirchhoff. Radiation strictly and
necessarily corresponds with absorption only _when the temperature is
the same_. In point of fact, Ångström was still, in 1853, divided
between adsorption and interference as the mode of origin of the
Fraunhofer dark rays. Very important, however, was his demonstration of
the compound nature of the spark-spectrum, which he showed to be made up
of the spectrum of the metallic electrodes superposed upon that of the
gas or gases across which the discharge passed.
It may here be useful--since without some clear ideas on the subject no
proper understanding of recent astronomical progress is possible--to
take a cursory view of the elementary principles of spectrum analysis.
To many of our readers they are doubtless already familiar; but it is
better to appear trite to some than obscure even to a few.
The spectrum, then, of a body is simply the light proceeding from it
_spread out_ by refraction[399] into a brilliant variegated band,
passing from brownish-red through crimson, orange, yellow, green, and
azure into dusky violet. The reason of this spreading-out or
"dispersion" is that the various colours have different wave-lengths,
and consequently meet with different degrees of retardation in
traversing the denser medium of the prism. The shortest and quickest
vibrations (producing the sensation we call "violet") are thrown
farthest away from their original path--in other words, suffer the
widest "deviation;" the longest and slowest (the red) travel much nearer
to it. Thus the sheaf of rays which would otherwise combine into a patch
of white light are separated through the divergence of their tracks
after refraction by a prism, so as to form a tinted riband. This
_visible_ spectrum is prolonged _invisibly_ at both ends by a long range
of vibrations, either too rapid or too sluggish to affect the eye as
light, but recognisable through their chemical and heating effects.
Now all incandescent solid or liquid substances, and even gases ignited
under great pressure, give what is called a "continuous spectrum;" that
is to say, the light derived from them is of every conceivable hue.
Sorted out with the prism, its tints merge imperceptibly one into the
other, uninterrupted by any dark spaces. No colours, in short, are
missing. But gases and vapours rendered luminous by heat emit rays of
only a few tints, which accordingly form an interrupted spectrum,
usually designated as one of lines or bands. And since these rays are
perfectly definite and characteristic--not being the same for any two
substances--it is easy to tell what kind of matter is concerned in
producing them. We may suppose that the inconceivably minute particles
which by their rapid thrilling agitate the ethereal medium so as to
produce light, are free to give out their peculiar tone of vibration
only when floating apart from each other in gaseous form; but when
crowded together into a condensed mass, the clear ring of the
distinctive note is drowned, so to speak, in a universal molecular
clang. Thus prismatic analysis has no power to identify individual kinds
of matter, except when they present themselves as glowing vapours.
A spectrum is said to be "reversed" when lines previously seen bright on
a dark background appear dark on a bright background. In this form it is
equally characteristic of chemical composition with the "direct"
spectrum, being due to _absorption_, as the latter is to _emission_. And
absorption and emission are, by Kirchhoff's law, strictly correlative.
This is easily understood by the analogy of sound. For just as a
tuning-fork responds to sound-waves of its own pitch, but remains
indifferent to those of any other, so those particles of matter whose
nature it is, when set swinging by heat, to vibrate a certain number of
times in a second, thus giving rise to light of a particular shade of
colour, appropriate those same vibrations, and those only, when
transmitted past them,--or, phrasing it otherwise, are opaque to them,
and transparent to all others.
It should further be explained that the _shape_ of the bright or dark
spaces in the spectrum has nothing whatever to do with the nature of the
phenomena. The "lines" and "bands" so frequently spoken of are seen as
such for no other reason than because the light forming them is admitted
through a narrow, straight opening. Change that opening into a fine
crescent or a sinuous curve, and the "lines" will at once appear as
crescents or curves.
Resuming in a sentence what has been already explained, we find that the
prismatic analysis of the heavenly bodies was founded upon three classes
of facts: First, the unmistakable character of the light given by each
different kind of glowing vapour; secondly, the identity of the light
absorbed with the light emitted by each; thirdly, the coincidence
observed between rays missing from the solar spectrum and rays absorbed
by various terrestrial substances. Thus, a realm of knowledge,
pronounced by Morinus[400] in the seventeenth century, and no less
dogmatically by Auguste Comte[401] in the nineteenth, hopelessly out of
reach of the human intellect, was thrown freely open, and the chemistry
of the sun and stars took at once a leading place among the experimental
sciences.
The immediate increase of knowledge was not the chief result of
Kirchhoff's labours; still more important was the change in the scope
and methods of astronomy, which, set on foot in 1852 by the detection of
a common period affecting at once the spots on the sun and the magnetism
of the earth, was extended and accelerated by the discovery of spectrum
analysis. The nature of that change is concisely indicated by the
heading of the present chapter; we would now ask our readers to
endeavour to realise somewhat distinctly what is implied by the
"foundation of astronomical physics."
Just three centuries ago, Kepler drew a forecast of what he called a
"physical astronomy"--a science treating of the efficient causes of
planetary motion, and holding the "key to the inner astronomy."[402]
What Kepler dreamed of and groped after, Newton realized. He showed the
beautiful and symmetrical revolutions of the solar system to be governed
by a uniformly acting cause, and that cause no other than the familiar
force of gravity, which gives stability to all our terrestrial
surroundings. The world under our feet was thus for the first time
brought into physical connection with the worlds peopling space, and a
very tangible relationship was demonstrated as existing between what
used to be called the "corruptible" matter of the earth and the
"incorruptible" matter of the heavens.
This process of unification of the cosmos--this levelling of the
celestial with the sublunary--was carried no farther until the fact
unexpectedly emerged from a vast and complicated mass of observations,
that the magnetism of the earth is subject to subtle influences,
emanating, certainly from some, and presumably from all of the heavenly
bodies; the inference being thus rendered at least plausible, that a
force not less universal than gravity itself, but with whose modes of
action we are as yet unacquainted, pervades the universe, and forms, it
might be said, an intangible bond of sympathy between its parts. Now for
the investigation of this influence two roads are open. It may be
pursued by observation either of the bodies from which it proceeds, or
of the effects which it produces--that is to say, either by the
astronomer or by the physicist, or, better still, by both concurrently.
Their acquisitions are mutually profitable; nor can either be considered
as independent of the other. Any important accession to knowledge
respecting the sun, for example, may be expected to cast a reflected
light on the still obscure subject of terrestrial magnetism; while
discoveries in magnetism or its _alter ego_ electricity must profoundly
affect solar inquiries.
The establishment of the new method of spectrum analysis drew far closer
this alliance between celestial and terrestrial science. Indeed, they
have come to merge so intimately one into the other, that it is no
easier to trace their respective boundaries than it is to draw a clear
dividing-line between the animal and vegetable kingdoms. Yet up to the
middle of the last century, astronomy, while maintaining her strict
union with mathematics, looked with indifference on the rest of the
sciences; it was enough that she possessed the telescope and the
calculus. Now the materials for her inductions are supplied by the
chemist, the electrician, the inquirer into the most recondite mysteries
of light and the molecular constitution of matter. She is concerned with
what the geologist, the meteorologist, even the biologist, has to say;
she can afford to close her ears to no new truth of the physical order.
Her position of lofty isolation has been exchanged for one of community
and mutual aid. The astronomer has become, in the highest sense of the
term, a physicist; while the physicist is bound to be something of an
astronomer.
This, then, is what is designed to be conveyed by the "foundation of
astronomical or cosmical physics." It means the establishment of a
science of Nature whose conclusions are not only presumed by analogy,
but are ascertained by observation, to be valid wherever light can
travel and gravity is obeyed--a science by which the nature of the stars
can be studied upon the earth, and the nature of the earth can be made
better known by study of the stars--a science, in a word, which is, or
aims at being, one and universal, even as Nature--the visible reflection
of the invisible highest Unity--is one and universal.
It is not too much to say that a new birth of knowledge has ensued. The
astronomy so signally promoted by Bessel[403]--the astronomy placed by
Comte[404] at the head of the hierarchy of the physical sciences--was
the science of the _movements_ of the heavenly bodies. And there were
those who began to regard it as a science which, from its very
perfection, had ceased to be interesting--whose tale of discoveries was
told, and whose farther advance must be in the line of minute technical
improvements, not of novel and stirring disclosures. But the science of
the _nature_ of the heavenly bodies is one only in the beginning of its
career. It is full of the audacities, the inconsistencies, the
imperfections, the possibilities of youth. It promises everything; it
has already performed much; it will doubtless perform much more. The
means at its disposal are vast and are being daily augmented. What has
so far been secured by them it must now be our task to extricate from
more doubtful surroundings and place in due order before our readers.
FOOTNOTES:
[Footnote 347: Wolf, _Gesch. der Astr._, p. 655.]
[Footnote 348: Manuel Johnson, _Mem. R.A.S._, vol. xxvi., p. 197.]
[Footnote 349: _Astronomie Théorique et Pratique_, t. iii., p. 20.]
[Footnote 350: Wolf, _Gesch. der Astr._, p. 654.]
[Footnote 351: _Month. Not._, vol. xvii., p. 241.]
[Footnote 352: _Mem. R.A.S._, vol. xxvi., p. 200.]
[Footnote 353: _Astr. Nach._, No. 495.]
[Footnote 354: Gehler's _Physikalisches Wörterbuch_, art.
_Sonnenflecken_, p. 851.]
[Footnote 355: _Zweite Abth._, p. 401.]
[Footnote 356: _Annalen der Physik_ (Poggendorff's), Bd. lxxxiv., p.
580.]
[Footnote 357: _Phil. Trans._, vol. cxlii., p. 103.]
[Footnote 358: _Mittheilungen der Naturforschenden Gesellschaft_, 1852,
p. 183.]
[Footnote 359: _Archives des Sciences_, t. xxi., p. 194.]
[Footnote 360: _Neue Untersuchungen, Mitth. Naturf. Ges._, 1852, p.
249.]
[Footnote 361: _Phil. Trans._, vol. xci., p. 316.]
[Footnote 362: Evidence of an eleven-yearly fluctuation in the price of
food-grains in India was collected some years ago by Mr. Frederick
Chambers. _Nature_, vol. xxxiv., p. 100.]
[Footnote 363: _Bibl. Un. de Genève_, t. li., p. 336.]
[Footnote 364: _Neue Untersuchungen_, p. 269.]
[Footnote 365: _Die Sonne und ihre Flecken_, p. 30. Arago was the first
who attempted to decide the question by keeping, through a series of
years, a parallel register of sun-spots and weather; but the data
regarding the solar condition amassed at the Paris Observatory from 1822
to 1830 were not sufficiently precise to support any inference.]
[Footnote 366: _Phil. Trans._, vol. xxix., p. 421.]
[Footnote 367: _Ibid._, vols. cxliii., p. 558, cxlvi., p. 505.]
[Footnote 368: _Observations on Light and Colours_, p. 35.]
[Footnote 369: _Phil. Trans._, vol. lxxv., p. 190.]
[Footnote 370: _Denkschriften_ (Munich. Ac. of Sc.), 1814, 1815, Bd. v.,
p. 197.]
[Footnote 371: _Edinburgh Journal of Science_, vol. v., p. 77. See also
_Phil. Mag._, Feb., 1834, vol. iv., p. 112.]
[Footnote 372: _Ed. Phil. Trans.,_ vol. xxi., p. 411.]
[Footnote 373: _On the Absorption of Light by Coloured Media, Ed. Phil.
Trans._, vol. ix., p. 445 (1823).]
[Footnote 374: _Phil. Mag._, vol. xxvii, (ser. iii.), p. 81.]
[Footnote 375: _Report Brit. Ass._, 1835, p. 11 (pt. ii.). _Electrodes_
are the terminals from one to the other of which the electric spark
passes, volatilising and rendering incandescent in its transit some
particles of their substance, the characteristic light of which
accordingly flashes out in the spectrum.]
[Footnote 376: _Phil. Mag._, vol. xx., p. 93.]
[Footnote 377: _Annalen der Physik_, Bd. cxiii., p. 357.]
[Footnote 378: _Phil. Trans._, vol. xcii., p. 378.]
[Footnote 379: _Denkschriften_, Bd. v., p. 202.]
[Footnote 380: _Ibid._, p. 220; _Edin. Jour. of Science_, vol. viii., p.
9.]
[Footnote 381: _Denkschriften_, Bd. v., p. 222.]
[Footnote 382: _Arch. des Sciences_, 1849, p. 43.]
[Footnote 383: _Phil. Trans._, vol. cl., p. 159, _note_.]
[Footnote 384: _Ed. Phil. Trans._, vol. xii., p. 528.]
[Footnote 385: _Phil. Trans._, vol. cxxvi., p. 453. "I conceive," he
says, "that this result proves decisively that the sun's atmosphere has
nothing to do with the production of this singular phenomenon" (p. 455).
And Brewster's well-founded opinion that it had much to do with it was
thereby, in fact, overthrown.]
[Footnote 386: _Monatsberichte_, Berlin, 1859, p. 664.]
[Footnote 387: _Abhandlungen_, Berlin, 1861, pp. 80, 81.]
[Footnote 388: _Ibid._, 1861, p. 77; _Annalen der Physik_, Bd. cxix., p.
275. A similar conclusion, reached by Balfour Stewart in 1858, for
heat-rays (_Ed. Phil. Trans._, vol. xxii., p. 13), was, in 1860, without
previous knowledge of Kirchhoff's work, extended to light (_Phil. Mag._,
vol. xx., p. 534); but his experiments wanted the precision of those
executed at Heidelburg.]
[Footnote 389: _Miscellaneous Works_, vol. i., p. 189.]
[Footnote 390: _Ed. Phil. Trans._, vol. ix., p. 458.]
[Footnote 391: _Ibid._, vol. xii., p. 519.]
[Footnote 392: _Quart. Jour. Chem. Soc._, vol. x. p. 79.]
[Footnote 393: A facsimile accompanied Sir H. Roscoe's translation of
Kirchhoff's "Researches on the Solar Spectrum" (London, 1862-63).]
[Footnote 394: Estimated by Kirchhoff's at a _trillion to one_.
_Abhandl._, 1861, p. 79.]
[Footnote 395: _Phil. Mag._, vol. xxvii. (3rd series), p. 90.]
[Footnote 396: _L'Institut_, Feb. 7, 1849, p. 45; _Phil. Mag._, vol.
xix. (4th series), p. 193.]
[Footnote 397: _Ann. d. Phys._, vol. cxviii., p. 110.]
[Footnote 398: _Phil. Mag._, vol. ix. (4th series), p. 327.]
[Footnote 399: Spectra may be produced by _diffraction_ as well as by
_refraction_; but we are here only concerned with the subject in its
simplest aspect.]
[Footnote 400: _Astrologia Gallica_ (1661), p. 189.]
[Footnote 401: _Pos. Phil._, vol. i., pp. 114, 115 (Martineau's
trans.).]
[Footnote 402: _Proem Astronomiæ Pars Optica_ (1640), _Op._, t. ii.]
[Footnote 403: _Pop. Vorl._, pp. 14, 19, 408.]
[Footnote 404: _Pos. Phil._, p. 115.]