THE INFLUENCE OF HERSCHEL’S CAREER ON MODERN ASTRONOMY.
The powers of the telescope were so unexpectedly increased, that they
may almost be said to have been discovered by William Herschel. No
one before him had considered the advantages of large apertures. No
one had seemed to remember that the primary function of an instrument
designed to aid vision is to collect light. The elementary principle
of space-penetration had not been adverted to. It devolved upon
him to point out that the distances of similar objects are exactly
proportional to the size of the telescopes barely sufficing to show
them. The reason is obvious. Compare, for instance, a one-inch
telescope with the naked eye. The telescope brings to a focus
twenty-five times as much light as can enter the pupil, taken at
one-fifth of an inch in diameter; therefore it will render visible
a star twenty-five times fainter than the smallest seen without its
help; or--what comes to the same thing--an intrinsically equal star
at a five-fold distance. A one-inch glass hence actually quintuples
the diameter of the visible universe, and gives access to seventy-five
times the volume of space ranged through by the unassisted eye.
This simple law Herschel made the foundation-stone of his sidereal
edifice. He was the first to notice it, because he was the first
practically to concern himself with the star-depths. The possibility
of gauging the heavens rose with him above the horizon of science.
Because untiring in exploration, he was insatiable of light; and being
insatiable of light, he built great telescopes.
His example was inevitably imitated and surpassed. Not through a vulgar
ambition to “beat the record,” but because a realm had been thrown open
which astronomers could not but desire to visit and search through for
themselves. Lord Rosse’s six-foot reflector was the immediate successor
of Herschel’s four-foot; Mr. Lassell’s beautiful specula followed; and
the series of large _metallic_ reflectors virtually closed with that of
four-feet aperture erected at Melbourne in 1870. The reflecting surface
in modern instruments is furnished by a thin film of silver deposited
on glass. It has the advantage of returning about half as much again of
the incident light as the old specula, so that equal power is obtained
with less size. Dr. Common’s five-foot is the grand exemplar in this
kind; and it is fully equivalent to the Parsonstown six-foot.
The improvement of refractors proceeded more slowly. Difficulties
in the manufacture of glass stood in the way, and difficulties in
the correction of colour. The splendid success, however, of the Lick
thirty-six inch, and the fine promise of the Yerkes forty-inch, have
turned the strongest current of hope for the future in the direction of
this class of instrument. But all modern efforts to widen telescopic
capacity primarily derive their impulse from Herschel’s passionate
desire to see further, and to see better, than his predecessors.
His observations demonstrate the rare excellence of his instruments.
Experiments made on the asteroid Juno, in 1805, for the purpose
of establishing a valid distinction between real and fictitious
star-discs, prove, in Professor Holden’s opinion, the reflector
employed to have been of almost ideal perfection; and his following of
Saturn’s inner satellites right up to the limb, with the twenty-foot
and the forty-foot, was a _tour de force_ in vision scarcely, if ever,
surpassed.
In the ordinary telescopes of those days really good definition was
unknown; they showed the stars with rays or tails, distorted into
triangles, or bulged into “cocked hats;” clean-cut, circular images
were out of the question. Sitting next Herschel one day at dinner,
Henry Cavendish, the great chemist, a remarkably taciturn man, broke
silence with the abrupt question--“Is it true, Dr. Herschel, that you
see the stars round?” “Round as a button,” replied the Doctor; and no
more was said until Cavendish, near the close of the repast, repeated
interrogatively, “Round as a button?” “Round as a button,” Herschel
briskly reiterated, and the conversation closed.
It seems probable that Herschel’s _caput artis_ lost some of its
fine qualities with time. Great specula are peculiarly liable to
deterioration. Their figure tends to become impaired by the stress of
their own weight; their lustre is necessarily more or less evanescent.
Re-polishing, however, is a sort of re-making; and the last felicitous
touches, upon which everything depends, can never be reckoned upon with
certainty. Hence, the original faultlessness of the great mirror was,
perhaps, never subsequently reproduced.
“Such telescopes as Herschel worked with,” Dr. Kitchiner wrote in
1815, “could only be made by the man who used them, and only be used
by the man who made them.” The saying is strictly true. His skill
in one branch promoted his success in the other. He was as much at
home with his telescopes as the Bedouin are with their horses. Their
peculiarities made part of his most intimate experience. From the
graduated varieties of his specula he picked out the one best suited
to the purpose in hand. It was his principle never to employ a larger
instrument than was necessary, agility of movement being taken into
account no less than capacity for collecting light. The time-element,
indeed, always entered into his calculations; he worked like a man who
has few to-morrows.
His sense of sight was exceedingly refined, and he took care to keep it
so. In order to secure complete “tranquility of the retina,” he used to
remain twenty minutes in the dark before attempting to observe faint
objects; and his eye became so sensitive after some hours spent in
“sweeping,” that the approach of a third-magnitude star obliged him to
withdraw it from the telescope. A black hood thrown over his head while
observing served to heighten this delicacy of vision. He despised no
precaution. Details are “of consequence,” he wrote to Alexander Aubert,
an amateur astronomer, “when we come to refinements, and want to _screw
an instrument up to the utmost pitch_.”
This was said in reference to his application of what seemed
extravagantly high magnifying powers. He laid great stress upon it in
the earlier part of his career. The method, he said, was “an untrodden
path,” in which “a variety of new phenomena may be expected.” With
his seven-foot Newtonian he used magnifications up to nearly 6,000,
proceeding, however, “all along experimentally”--a plan far too much
neglected in “the art of seeing.” “We are told,” he proceeded, “that
we gain nothing by magnifying _too much_. I grant it, but shall never
believe I magnify too much till by experience I find that I can see
better with a lower power.” The innovation was received with a mixture
of wonder, incredulity, and admiration.
Herschel showed his customary judgment in this branch of astronomical
practice. He established the distinctions still maintained, and
laid down the lines still followed. It is true he went far beyond
the point where modern observers find it advisable to stop. The
highest power brought into use with the Lick refractor is 2,600; and
Herschel’s instruments bore 5,800 (nominally 6,500) without injury to
definition. But only at exceptional moments. His habitual sweeping
power was 460; he “screwed-up” higher only for particular purposes,
and under favourable conditions. Although his strong eye-pieces seem,
for intelligible reasons, to have been laid aside on the adoption of
the “front-view” form of construction, they had served him well in
the division of close pairs, as well as for bringing faint stars into
view--an effect correctly explained by him as due to the augmented
darkness, under high powers, of the sky-ground. But the most important
result of their employment was the discovery that the stars have
no sensible dimensions. This became evident through the failure of
attempts to magnify them; the higher the power applied, the smaller and
more intense they appeared. Herschel accordingly pronounced stellar
telescopic discs “spurious,” but made no attempt to explain their
origin through diffraction.
He never possessed an instrument mounted equatoreally--that is, so as
automatically to follow the stars. In its absence, his work, had it not
been accomplished, would have seemed to modern ideas impossible. No
clockwork movement kept the objects he was observing in the field of
view. His hands were continually engaged in supplying the deficiency.
How, under these circumstances, he contrived to measure hundreds of
double stars, and secure the places of thousands of nebulæ, would be
incomprehensible but for the quasi-omnipotence of enthusiasm.
The angle made with the meridian by the line joining two stars (their
“position angle”) was never thought of as a quantity useful to be
ascertained until Herschel, about 1779, invented his “revolving-wire
micrometer.” This differed in no important respect from the modern
“filar micrometer;” only spider-lines have been substituted for the
original silk fibres. For measuring the distances of the wider classes
of double stars, he devised in 1782 a “lamp-micrometer;” while those
of the closest pairs were estimated in terms of the discs of the
components. In compiling his second catalogue, however, he used the
thread-micrometer for both purposes. It is true that “even in his
matchless hands”--in Dr. Gill’s phrase--the results obtained were
“crude;” but the fact remains that the whole system of micrometrical
measurement came into existence through Herschel’s double-star
determinations.
Their consequences have developed enormously within the last few
years. Mr. Burnham’s discoveries of excessively close pairs have been
so numerous as to leave no reasonable doubt that their indefinite
multiplication is only a question of telescopic possibility. Then
in 1889, another power came into play; the spectroscope took up the
work of resolving stars. Or rather, the spectroscope in alliance
with the photographic camera; for the spectral changes indicating
the direction and velocity of motion in the line of sight can be
systematically studied, as a rule, only when registered on sensitive
plates. The upshot has been to bring within the cognisance of science
the marvellous systems known as “spectroscopic binaries.” They are of
great variety. Some consist of a bright, others of a bright and dark,
pair. Those that revolve in a plane nearly coinciding with our line
of vision undergo mutual occultations. A further detachment seem to
escape eclipse, yet vary in light for some unexplained reason, while
they revolve. Others, like Spica Virginis, revolve without varying.
Their orbital periods are counted by hours or days. The study of the
disturbances of these remarkable combinations promises to open a new
era in astronomical theory. For they are most likely all multiple.
Irregularities indicating the presence of attractive, although obscure
bodies, have, in several cases, been already noticed.
The revolutions of spectroscopic binary stars can be studied to the
greatest advantage when they involve light-change; and photometric
methods have accordingly begun to play an important part in the
sidereal department of gravitational science. And here again we meet
with Herschel’s initiative. His method of sequences has been already
explained; and he made the first attempt to lay down a definite scale
of star-magnitudes. He failed, and it was hardly desirable that he
should succeed. On his scale, the ratio of change from one grade to the
next constantly diminished. In the modern system it remains always the
same. A star of the second magnitude is by definition two and a-half
(2·512) times less bright than one of the first; a star of the third
magnitude is two and a-half times less bright than one of the second,
the series descending without modification until beyond telescopic
reach. This uniformity in the _proportionate_ value of a magnitude
is indispensable for securing a practicable standard of measurement.
Herschel, however, took the great step of introducing a principle of
order.
His estimates of stellar lustre were purely visual. And although
various instruments, devised for the purpose, have since proved
eminently useful, the ultimate appeal in all is to the eye. But there
are many signs that, in the photometry of the future, not the eye but
the camera will be consulted. Their appraisements differ markedly.
Herschel’s incidental remark on the disturbance of light-valuation
by colour touches a point of fundamental importance in photographic
photometry. The chemical method gives to white stars a great advantage
over yellow and red ones. They come out proportionately much brighter
on the sensitive plate than they appear to the eye. And to these
varieties of hue correspond spectral class-distinctions, the spectrum
of an object being nothing but its colour written at full length. This
systematic discrepancy between visual and photographic impressions of
brightness, while introducing unwelcome complications in measures of
magnitude, may serve to bring out important truths. The inference, for
example, has been founded upon it that the Milky Way is composed almost
exclusively of white, or “Sirian” stars; and there can be no question
but that the arrangement of stars in space has some respect to their
spectral types.
Herschel’s plan of inquiry into the laws of stellar distribution by
“photometric enumeration,” or gauging by magnitudes, was a bequest
to posterity which has been turned to account with very little
acknowledgment of its source. Argelander’s review of the northern
heavens (lately completed photographically by Dr. Gill to the southern
pole) afforded, from 1862, materials for its application on a large
scale; but the magnitudes assigned to his 324,000 stars do not possess
the regularity needed to make deductions based on them perfectly
trustworthy. Otherwise the distance from the earth of the actual
aggregations in the Milky Way could have been ascertained in a rough
way from the numerical representation of the various photometric
classes. As it is, the presumption is strong that the galactic clouds
are wholly independent of stars brighter than the ninth magnitude--that
they only begin to gather at a depth in space whence light takes _at
least_ a thousand years to travel to our eyes. Confirmatory evidence,
published in 1894, has been supplied by M. Easton’s research, based on
the same principle, into the detailed relations of stars of various
magnitudes to Milky Way structure. They are exhibited only by those of
the ninth magnitude, or fainter; for with them sets in a significant
crowding upon its condensed parts, attended by a scarcity over its
comparative vacuities. Counts by magnitudes have, besides, made it
clear that the stars, in portions of the sky removed from the Milky
Way, thin out notably before the eleventh magnitude is reached; so
that, outside the galactic zone, the stellar system is easily fathomed.
Also on the strength of photometric enumerations, Dr. Gould, of Boston,
came to the conclusion, in 1879, that there is an extra thronging of
stars about our sun, which forms one of a special group consisting
of some four or five hundred members. The publication, in 1890, of
the “Draper Catalogue,” of 10,530 photographed stellar spectra, has
thrown fresh light on this interesting subject. Mr. Monck, of Dublin,
gave reasons for holding stars physically like the sun to be generally
nearer to us than stars of the Sirian class; and Professor Kapteyn, of
Gröningen, as the result of a singularly able investigation, concluded
with much probability that the sun belongs to a strongly condensed
group of mostly “solar” stars, nearly concentric with the galaxy. It
might, in fact, be said that we live in a globular cluster, since our
native star-collection should appear from a very great distance under
that distinctive form.
This modern quasi-discovery was anticipated by Herschel. He was
avowedly indebted, it is true, to Michell’s “admirable idea” of the
stars being divided into separate groups; but Michell did not trouble
himself about the means of its possible verification, and Herschel did.
He always looked round to see if there were not some touchstone of fact
within reach.
His discussion of the solar cluster, though brief and incidental, is
not without present interest. He found the federative arrangement of
the stars to be “every day more confirmed by observation.” The “flying
synods of worlds” formed by them must gravitate one towards another as
if concentrated at their several centres of gravity. Accordingly, “a
star, or sun, such as ours, may have a proper motion within its own
system of stars, while the whole may have another proper motion totally
different in quantity and direction.” We may thus, he continued,
“arrive in process of time, at a knowledge of all the real, complicated
motions of the planet we inhabit; of the solar system to which it
belongs; and even of the sidereal system of which the sun may possibly
be a member.” He proceeded to explain how stars, making part of the
solar cluster, might be discriminated from those exterior to it; the
former showing the perspective influence only of the sun’s translation
among themselves, while the latter would be affected besides by a
“still remoter parallax”--a secular drift, compounded of the proper
motion of the sun within its cluster, and of its cluster relatively to
other clusters.
The possibility of applying Herschel’s test is now fully recognised.
Each fresh determination of the solar apex is scrutinised for symptoms
of the higher “systematical parallax;” although as yet with dubious
or negative results. Associated stellar groups are, nevertheless, met
with in various parts of the sky. Herschel not only anticipated their
existence, but suggested “a concurrence of proper motions” as the
fittest means for identifying them.
His anticipation has been realised by Mr. Proctor’s detection of
“star-drift.” Several stars in the Plough thus form a squadron sailing
the same course; and similar combinations, on an apparently smaller
scale, have been pieced together in various constellations. But the
principle of their connection has yet to be discovered. They are
evidently not self-centred systems; hence their companionship, however
prolonged, must finally terminate. The only pronounced cluster with a
common proper motion is the Pleiades; and its drift seems to be merely
of a perspective nature--a reflection of the sun’s advance.
Bessel said of Herschel that “he aimed at acquiring knowledge, not of
the motions, but of the constitution of the heavenly bodies, and of
the structure of the sidereal edifice.” This, however, is a defective
appreciation. He made, indeed, no meridian observations, and computed
no planetary or cometary perturbations; yet if there ever was an
astronomer who instinctively “looked before and after,” it was he.
Could he have attained to a complete knowledge of the architecture
of the heavens, as they stood at a given moment, it would not have
satisfied him. To interpret the past and future by the present was his
constant aim; from his “retired situation” on the earth, he watched
with awe the grand procession of the sum of things defile through
endless ages. He could not observe what was without at the same time
seeking to divine what had been, and to forecast what was to come.
His nebular theory is now accepted almost as a matter of course. The
spectroscope has lent it powerful support by proving the _de facto_
existence of the “lucid medium,” postulated by him as a logical
necessity. This was done August 1st, 1864, when Dr. Huggins derived
from a planetary nebula in Draco a spectrum characteristic of a gaseous
body, because consisting of bright lines. Their wave-lengths, which
turned out to be identical for all objects of the kind, with one or
two possible exceptions, indicated a composition out of hydrogen
mixed with certain unfamiliar aeriform substances. Herschel’s visual
discrimination of gaseous nebulæ was highly felicitous. Modern science
agrees with him in pronouncing the Orion nebula, as well as others
of the irregular class, planetaries, diffused nebulosities, and the
“atmospheres” of “cloudy stars,” to be masses of “shining fluid.” As
for his “ambiguous objects,” they remain ambiguous still. “Clusters
in disguise” through enormous distance, give apparently the same
quality of light with irresolvable nebulæ. His inference that stars
and nebulæ form mixed systems has, moreover, been amply confirmed. No
one now denies their significant affinity, and very few their genetic
relationship.
Herschel gave a list in 1811 of fifty-two dim, indefinite nebulosities,
covering in the aggregate 152 square degrees. “But this,” he added,
“gives us by no means the real limits” of the luminous appearance;
“while the depth corresponding to its superficial extent may be far
beyond the reach of our telescopes;” so “that the abundance of nebulous
matter diffused through such an expansion of the heavens must exceed
all imagination.”
“The prophetic spirit of these remarks,” Professor Barnard comments,
“is being every day made more evident through the revelations of
photography.” He is himself one of the very few who have telescopically
verified any part of these suggestive observations.
“I am familiar,” he wrote in _Knowledge_, January, 1892, “with a number
of regions in the heavens where vast diffusions of nebulous matter are
situated. One of these, in a singularly blank region, lies some five
or six degrees north-west of Antares, and covers many square degrees.
Another lies north of the Pleiades, between the cluster and the Milky
Way; a portion of this has recently been successfully photographed
by Dr. Archenhold. There is a large nebulous spot in that region,
easily visible to the naked eye, which I have seen for many years. When
sweeping there with a low power, the whole region between the Pleiades
and the Milky Way is perceived to be nebulous. These great areas of
nebulosity make their presence known by a singular dulling of the
ordinarily black sky, as if a thin veil of dust intervened.” They “are
specially suitable for the photographic plate, and it is only by such
means that they can be at all satisfactorily located.”
Some of Herschel’s milky tracts have been thus pictured; notably one
in the Swan, shown on Dr. Max Wolf’s plates to involve the bright star
Gamma Cygni; and another immense formation extending over sixty square
degrees about the belt and sword of Orion, and joining on, Herschel was
“pretty sure,” to the great nebula. This, never unmistakably _seen_
except by him, portrayed itself emphatically in 1886 in Professor E.
C. Pickering’s photographs. Herschel’s persuasion of the subordinate
character of the original “Fish-mouth nebula” was well-grounded.
On plates exposed by Professors W. H. Pickering and Barnard, it is
disclosed as the mere nucleus of a tremendous spiral, sweeping round
from Bellatrix to Rigel.
Diffused nebulosities appear in photographs as far from homogeneous.
They are not simple volumes of gas indefinitely expanding in all
directions, after the manner of simple aeriform fluids. They possess,
on the contrary, characteristic shapes. Structureless nebulæ, like
structureless protoplasm, seem to be non-existent. In all, an
organising principle is at work.
Minute telescopic stars showed to Herschel as prevalently red, owing,
he conjectured, to the enfeeblement of their blue rays during an
uncommonly long journey through space “not quite destitute of some very
subtle medium.” The argument is a remarkable one. It would be valid if
the ethereal vehicle of light exercised absorption after the manner of
ordinary attenuated substances. There is, however, reason to suppose
that the symptomatic redness was only a subjective impression, not an
objective fact. His colour-sense was not quite normal. The lower, to
his perception, somewhat overbalanced the higher end of the spectrum,
and his mirrors added to the inequality by reflecting a diminished
proportion of blue light. Thus he recorded many stars as tinged with
red which are now colourless, yet lie under no suspicion of change.
Herschel was, in the highest and widest sense, the founder of sidereal
astronomy. He organised the science and set it going; he laid down
the principles of its future action; he accumulated materials for its
generalisations, and gave examples of how best to employ them. His work
was at once so stimulating and so practical that its abandonment might
be called impossible. Others were sure to resume where he had left
off. His son was his first and fittest successor; he was the only one
who undertook in its entirety the inherited task. Yet there are to be
found in every quarter of the world men imbued with William Herschel’s
sublime ambitions. Success swells the ranks of an invading army; and
the march of astronomy has, within the last decade, assumed a triumphal
character. The victory can never be completely won; the march can never
reach its final goal; but spoils are meanwhile gathered up by the
wayside which eager recruits are crowding in to share. The heavens
are, year by year, giving up secrets long and patiently watched for,
while holding in reserve many others still more mysterious. There is no
fear of interest being exhausted by disclosure.
Herschel’s dim intuition that something might be learned about the
physical nature of the stars from the diverse quality of their light,
was verified after sixty-five years, by the early researches of
Secchi, Huggins, and Miller; but he could not suspect that, through
the chemical properties, which he guessed to belong in varying degrees
to the different sections of their spectra, pictures of the heavenly
bodies would be obtained more perfect than the telescopic views he
rapturously gazed at. Still less could he have imagined that, owing
to its faculty of accumulating impressions too weak to affect the
eye separately, the chemical would, in great measure, supersede the
telescopic method in carrying out the designs he had most at heart.
Those designs have now grown to be of international importance. At
eighteen northern and southern observatories a photographic review
of the heavens is in progress. The combined results will be the
registration, in place and magnitude, of fifteen to twenty millions
of stars. The gauging of the skies will then be complete down to the
fourteenth magnitude; and the “construction of the heavens” can be
studied with materials of the best quality, and almost indefinite
in quantity. By simply “counting the gauges” on Herschel’s early
plan, much may be learnt; the amount of stellar condensation
towards the plane of the Milky Way, for instance, and the extent
of stellar denudation near its poles. A marked contrast between
the measures of distribution in these opposite directions will most
likely be brought into view. The application of his later method of
enumeration by magnitudes ought to prove even more instructive, but
may be very difficult. The obstacles, it is to be hoped, will not be
insurmountable; yet they look just now formidable enough.
The grand problem with which Herschel grappled all his life involves
more complicated relations than he was aware of. It might be compared
to a fortress, the citadel of which can only be approached after
innumerable outworks have been stormed. That one man, urged on by
the exalted curiosity inspired by the contemplation of the heavens,
attempted to carry it by a _coup de main_, and, having made no
inconsiderable breach in its fortifications, withdrew from the
assault, his “banner torn, but flying,” must always be remembered with
amazement.
[Illustration: CAROLINE LUCRETIA HERSCHEL.
(_From a portrait taken by Tielemann in 1829._)]