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[Illustration:
LONDON STEREOSCOPIC CO. PHOTOMEZZOTYPE.
STANMORE OBSERVATORY.
INSIDE VIEW.]
TELESCOPIC WORK
FOR
STARLIGHT EVENINGS.
BY
WILLIAM F. DENNING, F.R.A.S.
(FORMERLY PRESIDENT OF THE LIVERPOOL ASTRONOMICAL SOCIETY).
“To ask or search I blame thee not, for heaven
Is as the book of God before thee set,
Wherein to read his wondrous works.”
MILTON.
LONDON:
TAYLOR AND FRANCIS, RED LION COURT, FLEET STREET.
1891.
[_All rights reserved._]
[Illustration: ALERE FLAMMAM.]
PRINTED BY TAYLOR AND FRANCIS,
RED LION COURT, FLEET STREET.
PREFACE.
It having been suggested by some kind friends that a series of articles
on “Telescopes and Telescopic Work,” which I wrote for the ‘Journal of
the Liverpool Astronomical Society’ in 1887-8, should be reprinted, I
have undertaken the revision and rearrangement of the papers alluded
to. Certain other contributions on “Large and Small Telescopes,”
“Planetary Observations,” and kindred subjects, which I furnished to
‘The Observatory’ and other scientific serials from time to time, have
also been included, and the material so much altered and extended that
it may be regarded as virtually new matter. The work has outgrown my
original intention, but it proved so engrossing that it was found
difficult to ensure greater brevity.
The combination of different papers has possibly had the effect of
rendering the book more popular in some parts than in others. This
is not altogether unintentional, for the aim has been to make the
work intelligible to general readers, while also containing facts
and figures useful to amateur astronomers. It is merely intended as a
contribution to popular astronomy, and asserts no rivalry with existing
works, many of which are essentially different in plan. If any excuse
were, however, needed for the issue of this volume it might be found
in the rapid progress of astronomy, which requires that new or revised
works should be published at short intervals in order to represent
existing knowledge.
The methods explained are approximate, and technical points have
been avoided with the view to engage the interest of beginners who
may find it the stepping-stone to more advanced works and to more
precise methods. The object will be realized if observers derive any
encouragement from its descriptions or value from its references,
and the author sincerely hopes that not a few of his readers will
experience the same degree of pleasure in observation as he has done
during many years.
No matter how humble the observer, or how paltry the telescope,
astronomy is capable of furnishing an endless store of delight to its
adherents. Its influences are elevating, and many of its features
possess the charms of novelty as well as mystery. Whoever contemplates
the heavens with the right spirit reaps both pleasure and profit, and
many amateurs find a welcome relaxation to the cares of business in
the companionship of their telescopes on “starlight evenings.”
The title chosen is not, perhaps, a comprehensive one, but it covers
most of the ground, and no apology need be offered for dealing with one
or two important objects not strictly within its scope.
For many of the illustrations I must express my indebtedness to the
Editors of the ‘Observatory’ to the Council of the R.A.S., to the
proprietors of ‘Nature,’ to Messrs. Browning, Calver, Cooke & Sons,
Elger, Gore, Horne Thornthwaite and Wood, Klein, and other friends.
The markings on Venus and Jupiter as represented on pages 150 and 180
have come out much darker than was intended, but these illustrations
may have some value as showing the position and form of the features
delineated. It is difficult to reproduce delicate planetary markings
in precisely the same characters as they are displayed in a good
telescope. The apparent orbits of the satellites of the planets,
delineated in figs. 41, 44, &c., are liable to changes depending on
their variable position relatively to the Earth, and the diagrams are
merely intended to give a good idea of these satellite systems.
W. F. D.
Bishopston, Bristol,
1891.
Plates I. and II. are views of the Observatory and Instruments recently
erected by Mr. Klein at Stanmore, Middlesex, lat. 51° 36′ 57″ N.,
long. 0° 18′ 22″ W. The height above sea-level is 262 feet. The
telescope is a 20-inch reflector by Calver, of 92 inches focus; the
tube is, however, 152 inches long so as to cut off all extraneous
rays. It is mounted equatoreally, and is provided with a finder of
6 inches aperture—one of Tulley’s famous instruments a century ago.
The large telescope is fixed on a pillar of masonry 37 feet high, and
weighing 115 tons. Mr. Klein proposes to devote the resources of his
establishment to astronomical photography, and it has been provided
with all the best appliances for this purpose. The observatory is
connected by telephone with Mr. Klein’s private residence, and the
timepieces and recording instruments are all electrically connected
with a centre of observation in his study.
CONTENTS.
CHAPTER I.
Page
THE TELESCOPE, ITS INTENTION AND THE DEVELOPMENT OF ITS POWERS 1
CHAPTER II.
RELATIVE MERITS OF LARGE AND SMALL TELESCOPES 20
CHAPTER III.
NOTES ON TELESCOPES AND THEIR ACCESSORIES 38
CHAPTER IV.
NOTES ON TELESCOPIC WORK 66
CHAPTER V.
THE SUN 87
CHAPTER VI.
THE MOON 113
CHAPTER VII.
MERCURY 137
CHAPTER VIII.
VENUS 145
CHAPTER IX.
MARS 155
CHAPTER X.
THE PLANETOIDS 167
CHAPTER XI.
JUPITER 170
CHAPTER XII.
SATURN 195
CHAPTER XIII.
URANUS AND NEPTUNE 215
CHAPTER XIV.
COMETS AND COMET-SEEKING 227
CHAPTER XV.
METEORS AND METEORIC OBSERVATIONS 260
CHAPTER XVI.
THE STARS 286
CHAPTER XVII.
NEBULÆ AND CLUSTERS OF STARS 324
NOTES AND ADDITIONS 347
INDEX 353
ILLUSTRATIONS.
PLATE I. Interior of Mr. Klein’s Observatory _Frontispiece_
II. View of Mr. Klein’s Grounds and Observatory _To face_ p. 82
FIG. PAGE
1. The Galilean Telescope 7
2. Royal Observatory, Greenwich, in Flamsteed’s time 8
3. Sir Isaac Newton 10
4. Gregorian Telescope 10
5. Cassegrainian Telescope 11
6. Newtonian Telescope 11
7. Common Refracting-Telescope 12
8. Le Mairean or Herschelian Telescope 13
9. 10-inch Reflecting-Telescope on a German Equatoreal, by Calver 17
10. Lord Rosse’s 6-foot Reflecting-Telescope 22
11. Refracting-Telescope, by Browning 32
12. “The Popular Reflector,” by Calver 40
13. 3-inch Refracting-Telescope, by Newton & Co. 41
14. Huygens’s Negative Eyepiece 46
15. Ramsden’s Positive Eyepiece 47
16. Berthon’s Dynamometer 50
17. Cooke and Sons’ Educational Telescope 52
18. Refracting-Telescope on a German Equatoreal 67
19. The Author’s Telescope: a 10-inch With-Browning Reflector 77
20. Sun-spot of June 19, 1889 95
21. Solar Eclipses visible in England, 1891 to 1922 98
22. Total Solar Eclipse of August 19, 1887 98
23. Belts of Sun-spots, visible Oct. 29, 1868 104
24. Shadows cast by Faculæ 109
25. Light-spots and streaks on Plato, 1879-82. (A. Stanley
Williams.) 126
26. Petavius and Wrottesley at Sunset. (T. Gwyn Elger.) 129
27. Birt, Birt A, and the Straight Wall. (T. Gwyn Elger.) 130
28. Aristarchus and Herodotus at Sunrise. (T. Gwyn Elger.) 132
29. Mercury as a Morning Star 143
30. Venus as an Evening Star 150
31. Mars, 1886, April 13, 9^h 50^m 157
32. Orbits of the Satellites of Mars 159
33. Jupiter, as drawn by Dawes and others 178
34. Jupiter, 1886, April 9, 10^h 12^m 180
35. Occultation of Jupiter, Aug. 7, 1889 186
36. Jupiter and Satellites seen in a small glass 187
37. Shadows of Jupiter’s Satellites II. and III. 192
38. Saturn as observed by Cassini in August 1676 198
39. Saturn, 1885, Dec. 23, 7^h 54^m 201
40. Saturn as observed by F. Terby, February 1887 203
41. Apparent orbits of the Five Inner Satellites of Saturn 212
42. Transit of the Shadow of Titan 213
43. Uranus and his belts 218
44. Apparent orbits of the Satellites of Uranus 221
45. Apparent orbit of the Satellite of Neptune 224
46. Mars, Saturn, and Regulus in same field, Sept. 20, 1889 226
47. Comet 1862 III. (Aug. 19, 1862) 237
48. Sawerthal’s Comet, 1888 I. (March 25, Brooks) 237
49. Brooks’s Double Comet, Sept. 17, 1889 239
50. Pons’s Comet (1812). Telescopic view, 1884, Jan. 6 242
51. Ditto. Ditto, 1884, Jan. 21 242
52. Radiation of Meteors. (Shower of early Perseids, 1878) 263
53. Double Meteor. Curved Meteor. Fireball 265
54. Meteorite found in Chili in 1866 265
55. Meteorite which fell at Orgueil in 1864 265
56. Fireball of Nov. 23, 1877, 8^h 24^m (J. Plant.) 269
57. Flight of Telescopic Meteors seen by W. R. Brooks 272
58. Meteor of Dec. 28, 1888, 6^h 17^m 277
59. Large Meteor and streak seen at Jask 278
60. The Constellation Orion 289
61. Diagram illustrating the Measurement of Angles of Position 291
62. Double Stars 301
63. Trapezium in Orion as seen with the 36-inch refractor 319
64. Nebulæ and a Star-cluster 336
65. Nebula within a semicircle of stars 342
TELESCOPIC WORK
FOR
STARLIGHT EVENINGS.
CHAPTER I.
_THE TELESCOPE, ITS INVENTION AND THE DEVELOPMENT OF ITS POWERS._
The instrument which has so vastly extended our knowledge of the
Universe, which has enabled us to acquire observations of remarkable
precision, and supplied the materials for many sublime speculations in
Astronomy, was invented early in the seventeenth century. Apart from
its special application as a means of exploring the heavens with a
capacity that is truly marvellous, it is a construction which has also
been utilized in certain other departments with signal success. It
provided mankind with a medium through which to penetrate far beyond
the reach of natural vision, and to grasp objects and phenomena which
had either eluded detection altogether or had only been seen in dim and
uncertain characters. It has also proved a very efficient instrument
for various minor purposes of instruction and recreation. The invention
of the telescope formed a new era in astronomy; and though, with a
few exceptions, men were slow at first in availing themselves of its
far-seeing resources, scepticism was soon swept aside and its value
became widely acknowledged.
But though the telescope was destined to effect work of the utmost
import, and to reach a very high degree of excellence in after
times, the result was achieved gradually. Step by step its powers
were enlarged and its qualities perfected, and thus the stream of
astronomical discovery has been enabled to flow on, stimulated by every
increase in its capacity.
There is some question as to whom may be justly credited with
the discovery of its principles of construction. Huygens, in his
‘Dioptrics,’ remarks:—“I should have no hesitation in placing above all
the rest of mankind the individual who, solely by his own reflections,
without the aid of any fortuitous circumstances, should have achieved
the invention of the telescope.” There is reason to conclude, however,
that its discovery resulted from accident rather than from theory. It
is commonly supposed that Galileo Galilei is entitled to precedence;
but there is strong evidence to show that he had been anticipated.
In any case it must be admitted that Galilei[1] had priority in
successfully utilizing its resources as a means of observational
discovery; for he it was who, first of all men, saw Jupiter’s
satellites, the crescent form of Venus, the mountains and craters on
the Moon, and announced them to an incredible world.
It has been supposed, and not without some basis of probability, that a
similar instrument to the telescope had been employed by the ancients;
for certain statements contained in old historical records would
suggest that the Greek philosophers had some means of extending their
knowledge further than that permitted by the naked eye. Democritus
remarked that the Galaxy or “Milky Way” was nothing but an assemblage
of minute stars; and it has been asked, How could he have derived this
information but by instrumental aid? It is very probable he gained the
knowledge by inferences having their source in close observation; for
anyone who attentively studies the face of the sky must be naturally
led to conclude that the appearance of the “Milky Way” is induced by
immense and irregular clusterings of small stars. In certain regions
of the heavens there are clear indications of this: the eye is enabled
to glimpse some of the individual star-points, and to observe how they
blend and associate with the denser aggregations which give rise to the
milky whiteness of the Galaxy.
Refracting lenses, or “burning-glasses,” were known at a very early
period. A lens, roughly figured into a convex shape and obviously
intended for magnifying objects, has been recovered from the ruins
of Herculaneum, buried in the ejections from Vesuvius in the year 79
A.D. Pliny and others refer to lenses that burnt by refraction, and
describe globules of glass or crystal which, when exposed in the sun,
transmit sufficient heat to ignite combustible material. The ancients
undoubtedly used tubes in the conduct of their observations, but no
lenses seem to have been employed with them, and their only utility
consisted in the fact of their shutting out the extraneous rays of
light. But spectacles were certainly known at an early period. Concave
emeralds are said to have been employed by Nero in witnessing the
combats of the gladiators, and they appear to have been the same in
effect as the spectacles worn by short-sighted people in our own times.
But the ancients supposed that the emerald possessed inherent qualities
specially helpful to vision, rather than that its utility resulted
simply from its concavity of figure. In the 13th century spectacles
were more generally worn, and the theory of their construction
understood.
It is remarkable that the telescope did not come into use until so
long afterwards. Vague references were made to such an instrument, or
rather suggestions as to the possibilities of its construction, which
show that, although the principle had perhaps been conceived, the idea
was not successfully put into practice. Roger Bacon, who flourished in
the 13th century, wrote in his ‘_Opus Majus_’:—“Greater things may
be performed by refracted light, for, from the foregoing principles,
follows easily that the greatest objects may be seen very small, the
remote very near, and _vice versâ_. For we can give transparent bodies
such form and position with respect to the eye and the object that the
rays are refracted and bent to where we like, so that we, under any
angle, see the objects near or far, and in that manner we can, at a
great distance, read the smallest letters, and we can count atoms and
sand-grains, on account of the greatness of the angle under which they
are seen.”
Fracastor, in a work published at Venice in 1538, states:—“If we look
through two eye-lenses, placed the one upon the other, everything
will appear larger and nearer.” He also says:—“There are made certain
eye-lenses of such a thickness that if the moon or any other celestial
body is viewed through them they appear to be so near that their
distance does not exceed that of the steeples of public buildings.”
In other writings will also be found intimations as to the important
action of lenses; and it is hardly accountable that a matter so
valuable in its bearings was allowed to remain without practical
issues. The progressive tendency and the faculty of invention must
indeed have been in an incipient stage, and contrasts strongly with the
singular avidity with which ideas are seized upon and realized in our
own day.
Many important discoveries have resulted from pure accident; and it
has been stated that the first _bonâ fide_ telescope had its origin in
the following incident:—The children of a spectacle-maker, Zachariah
Jansen, of Middleberg, in Zealand, were playing with some lenses, and
it chanced that they arranged two of them in such manner that, to their
astonishment, the weathercock of an adjoining church appeared much
enlarged and more distinct. Having mentioned the curious fact to their
father, he immediately turned it to account, and, by fixing two lenses
on a board, produced the first telescope!
This view of the case is, however, a very doubtful one, and the
invention may with far greater probability be attributed to Hans
Lippersheim in 1608. Galilei has little claim to be considered in this
relation; for he admitted that in 1609 the news reached him that a
Dutchman had devised an appliance capable of showing distant objects
with remarkable clearness. He thereupon set to work and experimented
with so much aptitude on the principles involved that he very soon
produced a telescope for himself. With this instrument he detected
the four satellites of Jupiter in 1610, and other successes shortly
followed. Being naturally gratified with the improvements he had
effected in its construction, and with the wonderful discoveries he had
made by its use, we can almost excuse the enthusiasm which prompted him
to attribute the invention to his own ingenuity. But while according
him the honour due to his sagacity in devoting this instrument to
such excellent work, we must not overlook the fact that his claim to
priority cannot be justified. Indeed, that Galilei had usurped the
title of inventor is mentioned in letters which passed between the
scientific men of that time. Fuccari, writing to Kepler, says:—“Galileo
wants to be considered the inventor of the telescope, though he, as
well as I and others, first saw the telescope which a certain Dutchman
first brought with him to Venice, and although he has only improved it
very little.”
In a critical article by Dr. Doberck[2], in which this letter is quoted
and the whole question reviewed with considerable care, it is stated
that Hans Lippersheim (also known as Jan Lapprey), who was born in
Wesel, but afterwards settled at Middleberg, in the Netherlands, as
a spectacle-maker, was really the first to make a telescope, and the
following facts are quoted in confirmation:—“He solicited the States,
as early as the 2nd October, 1608, for a patent for thirty years,
or an annual pension for life, for the instrument he had invented,
promising then only to construct such instruments for the Government.
After inviting the inventor to improve the instrument and alter it so
that they could look through it with both eyes at the same time, the
States determined, on the 4th October, that from every province one
deputy should be elected to try the apparatus and make terms with him
concerning the price. This committee declared on the 6th October that
it found the invention useful for the country, and had offered the
inventor 900 florins for the instrument. He had at first asked 3000
florins for three instruments of rock-crystal. He was then ordered
to deliver the instrument within a certain time, and the patent was
promised him on condition that he kept the invention secret. Lapprey
delivered the instrument in due time. He had arranged it for both
eyes, and it was found satisfactory; but they forced him, against the
agreement, to deliver two other telescopes for the same money, and
refused the patent because it was evident that already several others
had learned about the invention.”
The material from which the glasses were figured appears to have been
quartz; and efforts were made to keep the invention a profound secret,
as it was thought it would prove valuable for “strategetical purposes.”
The cost of these primitive binoculars was about £75 each.
It is singular that, after being allowed to rest so long, the idea of
telescopic construction should have been carried into effect by several
persons almost simultaneously, and that doubts and disputes arose as
to precedence. The probable explanation is that to one individual
only priority was really due, but that, owing to the delays, the
secret could not be altogether concealed from two or three others
who recognized the importance of the discovery and at once entered
into competition with the original inventor. Each of these fashioned
his instrument in a slightly different manner, though the principle
was similar in all; and having in a great measure to rely upon his
individual faculties in completing the task, he considered himself in
the light of an inventor and put forth claims accordingly. Not only
were attempts made to assume the position of inventor, but there arose
fraudulent claimants to some of the discoveries which the instrument
effected in the hands of Galilei. Simon Marius, himself one of the very
first to construct a telescope and apply it to the examination of the
heavenly bodies, asserted that he had seen the satellites of Jupiter
on December 29, 1609, a few days before Galilei, who first glimpsed
them on January 7, 1610. Humboldt, in his ‘Physical Description of the
Heavens,’ definitely ascribes the discovery of these moons to Marius;
but other authorities uniformly reject the statement, and accord to
Galilei the full credit.
It is stated that Galilei’s first instrument magnified only three
times, but he so far managed to amplify its resources that he was
ultimately enabled to apply a power of 30. The lenses consisted of a
double-convex object-glass, and a small double-concave eye-glass placed
in front of the focal image formed by the object-glass. The ordinary
opera-glass is constructed on a similar principle.
[Illustration: Fig. 1.
The Galilean Telescope.]
The discoveries which Galilei effected with this crude and defective
instrument caused a great sensation at the time. He made them known
through the medium of a publication which he issued under the title
of ‘_Nuncius Siderus_,’ or ‘The Messenger of the Stars.’ In that
superstitious age great ignorance prevailed, bigotry was dominant,
and erroneous views of the solar system were upheld and taught
by authority. We can therefore readily conceive that Galilei’s
discoveries, and the direct inferences he put upon them, being
held antagonistic to the ruling doctrines, would be received with
incredulity and opposition. His views were regarded as heretical. In
consequence of upholding the Copernican system he suffered persecution,
and had to resort to artifice in the publication of his works. But the
marvels revealed by his telescope, though discredited at first, could
not fail to meet with final acceptance, for undeniable testimony to
their reality was soon forthcoming. They were not, however, regarded
until long afterwards as affirming the views enunciated by their clever
author. Ultimately the new astronomy, based on the irrepressible
evidence of the telescope, and clad in all the habiliments of truth,
took the place of the old fallacious beliefs, to form an enduring
monument to Copernicus and Galilei, who spent their lives in advancing
its cause.
No special developments in the construction of the telescope appear
to have taken place until nearly half a century subsequent to its
invention. Kepler suggested an instrument formed of two convex
lenses, and Scheiner and Huygens made telescopes on this principle
in the middle of the 17th century. Huygens found great advantage
in the employment of a compound eyepiece consisting of two convex
lenses, which corrected the spherical aberration, and, besides being
achromatic, gave a much larger field than the single lens. This
eyepiece, known as the “Huygenian,” still finds favour with the makers
of telescopes.
[Illustration: Fig. 2.
Royal Observatory, Greenwich, in Flamsteed’s time[3].]
Huygens may be said to have inaugurated the era of _long_ telescopes.
He erected instruments of 12 and 23 feet, having an aperture of 2-1/3
inches and powers of 48, 50, and 92. He afterwards produced one
123 feet in focal length and 6 inches in aperture. Chief among his
discoveries were the largest satellite of Saturn (Titan) and the true
form of Saturn’s ring. Hevelius of Dantzic built an instrument 150 feet
long, which he fixed to a mast 90 feet in height, and regulated by
ropes and pulleys. Cassini, at the Observatory at Paris, had telescopes
by Campani of 86, 100, and 136 French feet in length; but the highest
powers he used on these instruments do not appear to have exceeded
150 times. He made such good use of them as to discover three of the
satellites of Saturn and the black division in the ring of that planet.
The largest object-glasses employed by Hevelius and Cassini were of
6, 7, and 8 inches diameter. This was during the latter half of the
17th century. In 1712 Bradley made observations of Venus, and obtained
measures of the planet’s diameter, with a telescope no less than 212
feet in focal length. The instruments alluded to were manipulated with
extreme difficulty, and observations had to be conducted in a manner
very trying to the observer. Tubes were sometimes dispensed with, the
object-glass being fixed to a pole and its position controlled by
various contrivances—the observer being so far off, however, that he
required the services of a good lantern in order to distinguish it!
The immoderate lengths of refracting-telescopes were necessary, as
partially avoiding the effects of chromatic aberration occasioned
by the different refrangibility of the seven coloured rays which
collectively make white light. In other words, the coloured rays having
various indices of refraction cannot be brought to a coincident focus
by transmission through a single lens. Thus the red rays have a longer
focus than the violet rays, and the immediate effect of the different
refractions becomes apparent in the telescopic images, which are
fringed with colour and not sharply defined. High magnifying powers
serve to intensify the obstacle alluded to, and thus the old observers
found it imperative to employ eye-glasses not beyond a certain degree
of convexity. The great focal lengths of their object-lenses enabled
moderate power to be obtained, though the eye-glass itself had a focus
of several inches and magnified very little.
Sir Isaac Newton made many experiments upon colours, and endeavoured
to obviate the difficulties of chromatic aberration, but erroneously
concluded that it was not feasible. He could devise no means to correct
that dispersion of colour which, in the telescopes of his day, so
greatly detracted from their effectiveness. His failure seems to have
had a prejudicial effect in delaying the solution of the difficulty,
which was not accomplished until many years afterwards.
[Illustration: Fig. 3.
Sir Isaac Newton[4].]
[Illustration: Fig. 4.
Gregorian Telescope.]
The idea of reflecting-telescopes received mention as early as 1639;
but it was not until 1663 that Gregory described the instrument,
formed of concave mirrors, which still bears his name. He was not,
however, proficient in mechanics, and after some futile attempts to
carry his theory into effect the exertion was relinquished. In 1673
Cassegrain revived the subject, and proposed a modification of the form
previously indicated by Gregory. Instead of the small concave mirror,
he substituted a convex mirror placed nearer the speculum; and this
arrangement, though it made the telescope shorter, had the disadvantage
of displaying objects in an inverted position. But the utility of these
instruments was not demonstrated in a practical form until 1674, when
Hooke, the clever mechanician, gave his attention to the subject and
constructed the first one that was made of the kind.
[Illustration: Fig. 5.
Cassegrainian Telescope.]
In the meantime (1672) Sir Isaac Newton had completed with his own
hands a reflecting-telescope of another pattern. In this the rays from
the large concave speculum were received by a small plane mirror fixed
centrally at the other end of the tube, and inclined at an angle of
45°; so that the image was directed at right angles through an opening
in the side, and there magnified by the eye-lens. But for a long period
little progress was effected in regard to reflecting-telescopes, owing
to the difficulty of procuring metal well adapted for the making of
specula.
[Illustration: Fig. 6.
Newtonian Telescope.]
In 1729 Mr. Chester Moor Hall applied himself to the study of
refracting-telescopes and discovered that, by a combination of
different glasses, the colouring of the images might be eliminated.
It is stated that Mr. Hall made several achromatic glasses in 1733. A
quarter of a century after this John Dollond independently arrived at
the same result, and took out a patent for achromatic telescopes. He
found, by experiments with prisms, that crown and flint glass operated
unequally in regard to the divergency of colours induced by refraction;
and, applying the principle further, he obtained a virtually colourless
telescope by assorting a convex crown lens with a concave flint lens
as the object-glass. Dollond also made many instruments having triple
object-lenses, and in these it was supposed that previous defects were
altogether obliterated. Two convex lenses of crown glass were combined
with a concave lens of flint glass placed between them.
Whether we regard Hall or Dollond as entitled to the most praise in
connection with this important advance, it is certain that it was
one the value of which could hardly be overestimated. It may be said
to have formed a new era in practical astronomy. Instruments only 4
or 5 feet long could now be made equally if not more effective than
those of 123 and 150 feet previously used by Huygens and Hevelius.
All the troubles incidental to these long unmanageable machines now
disappeared, and astronomers were at once provided with a handy little
telescope capable of the finest performances.
[Illustration: Fig. 7.
Common Refracting-Telescope.]
Reflecting-telescopes also underwent marked improvements in the
eighteenth century. Short, the optician, who died in 1768, was
deservedly celebrated for the excellent instruments he made of the
Gregorian form. Towards the latter part of the century William
Herschel, by indomitable perseverance, figured a considerable number
of specula. Some of these were mounted as Newtonians; others were
employed in the form known as the “Front view,” in which a second
mirror is dispensed with altogether, and the rays from the large
concave speculum are thrown to the side of the tube and direct to the
eyepiece. This construction is often mentioned as the “Herschelian,”
but the idea had long before been detailed by Le Maire. In 1728 he
presented a paper to the Académie des Sciences, giving his plans for
a new reflecting-telescope. He proposed to suppress the small flat
speculum in Newtonians, and “by giving the large concave speculum a
little inclination, he threw the image, formed in its focus, to one
side of the tube, where, an eye-glass magnifying it, the observer
viewed it, his back at the time being turned towards the object in the
heavens; thus the light lost in the Newtonian telescope by the second
reflexion was saved.”
[Illustration: Fig. 8.
The Le Mairean or Herschelian Telescope.]
After making several instruments of from 18 to 24 inches aperture,
Herschel began one of larger calibre, and it was finished on August
28, 1789. The occasion was rendered historical by the discovery of one
of the faintest interior satellites of Saturn, Enceladus. The large
telescope had a speculum 48 inches in diameter; the tube was made of
rolled or sheet iron, and it was 39 ft. 4 in. long and 4 ft. 10 in. in
diameter. It was by far the largest instrument the world had seen up
to that time; but it cannot be said to have realized the expectations
formed of its powers, for its defining properties were evidently
not on a par with its space-penetrating power. Many of Herschel’s
best observations were made with much smaller instruments. The large
telescope, which was mounted in Herschel’s garden at Slough, soon fell
into comparative disuse, and, regarding it as incapable of further
usefulness, Sir John Herschel sealed it up on January 1, 1840.
During the next half-century we hear of no attempts being made to
surpass the large instrument which formed one of the working-tools
of Herschel. Then, however, Lord Rosse entered the field, and in the
‘Philosophical Transactions’ for 1840 described a reflector of 3-feet
diameter which he had set up at his residence at Parsonstown, Ireland.
In 1845 the same nobleman, distinguished alike for his scientific
attainments as for his generosity and urbanity of disposition, erected
another telescope, the large speculum of which was 6 feet in diameter,
5½ inches in thickness, and its weight 3 tons. Lord Rosse subsequently
cast a duplicate speculum of 6 feet and weighing 4 tons. In point of
dimensions this instrument far exceeded that of Herschel, and it is
still in use, retaining its character as the largest, though certainly
not the best, telescope in existence. Its tube is made of 1-inch deal,
well bound together with iron hoops; it is 56 feet long and 7 feet in
diameter.
Mr. Lassell soon afterwards made large specula. He erected one of
2-feet aperture and 20-feet focus at his residence at Starfield, near
Liverpool, and in 1861 mounted one of 4-feet diameter and 37-feet
focus. This instrument was for some time usefully employed by him
at Malta. After Mr. Lassell’s return to England his great telescope
remained in a dismantled state for several years, and ultimately
the speculum was broken up and “consigned to the crucible of the
bell-founder.”
It is not a little remarkable that Herschel, Rosse, and Lassell
personally superintended and assisted in the construction of the
monster instruments with which their names are so honourably associated.
In or about the year 1867 a telescope of the Cassegrainian form, and
having a metallic speculum 4 feet in diameter and 28-feet focus, was
completed by Grubb of Dublin for the observatory at Melbourne. This
instrument, which cost something like £14,000, was found defective
at first, though the fault does not appear to have rested with the
optician.
Up to this period specula were formed of a metal in which copper and
tin were largely represented. But the days of metal specula were
numbered. Leon Foucault, in the year 1859, published a valuable memoir
in which he described the various ingenious methods he employed in
figuring surfaces of glass to the required curve. He furnished data for
determining accuracy of figure. Formerly opticians had considerable
trouble in deciding the quality of their newly-ground specula or
object-glasses. They found it expedient to mount them temporarily,
and then, by actual trial on difficult objects, to judge of their
efficiency. This involved labour and occasioned delay, especially in
the case of large instruments. Foucault showed that crucial tests might
be applied in the workshop, and that glasses could be turned out of
hand without any misgivings as to their perfection of figure.
Foucault’s early experiments in parabolizing glass led him to
important results. By depositing a thin coating of silver on his
specula he obtained a reflective power far surpassing that of metal.
Thereafter metal was not thought of as a suitable material for
reflecting-telescopes. Silver-on-glass mirrors immediately came into
great request. The latter undoubtedly possess a great superiority
over metal, especially as regards light-grasping power, the relative
capacity according to Sir J. Herschel being as ·824 to ·436. Glass
mirrors have also another advantage in being less heavy than those of
metal. It is true the silver film is not very durable, but it can be
renewed at any time with little trouble or expense.
With of Hereford, and after him Calver of Chelmsford, became noted
for the excellency of their glass mirrors. They were found nearly
comparable to refractors of the same aperture.
A tendency of the times was evidently in the direction of large
instruments. One of 47·2-inches aperture (for which a sum of 190,000
francs was paid) was completed by Martin in 1875 for the Paris
Observatory, but its employment since that year has not furnished a
very successful record. The largest instrument of the kind yet made has
a speculum 5 feet in diameter and 27½-feet focal length. It was placed
in position in September 1888, and was made by the owner, Mr. Common,
of Ealing, whose previous instrument was a 37-inch glass reflector by
Calver. The 5-foot telescope is undoubtedly of much greater capacity
than the colossal reflector of Lord Rosse, though it is not so large.
Mr. Calver has recently figured a 50-inch mirror for Sir H. Bessemer,
but the mounting is not completed; and he is expecting to make other
large reflectors, viz. one over 5 feet in diameter and another over
3 feet. The late Mr. Nasmyth also erected some fine instruments, and
adopted a combination of the Cassegrainian and Newtonian forms to
ensure greater convenience for the observer. Instead of permitting the
rays from the small convex mirror to return through the large mirror,
he diverted them through the side of the tube by means of a flat
mirror, as in Newtonians. But this construction is not to be commended,
because much light is lost and defects increased by the additional
mirror.
Smaller telescopes of the kind we have been referring to have become
extremely popular: and deservedly so. They are likely to maintain
their character in future years; for the Newtonian form of instrument,
besides being thoroughly effective in critical work, is moderate in
price and gives images absolutely achromatic. Moreover, it is used
with a facility and ease which an experienced observer knows how to
appreciate. Whatever may be the altitude of the objects under scrutiny,
he is enabled to retain a perfectly convenient and natural posture, and
may pursue his work during long intervals without any of the fatigue or
discomfort incidental to the use of certain other forms of instrument.
Returning now to refractors: many years elapsed after Dollond patented
his achromatic object-glass before it was found feasible to construct
these instruments of a size sufficient to grasp faint and delicate
objects. Opticians were thwarted in their efforts to obtain glass of
the requisite purity for lenses, unless in small disks very few inches
in diameter. It is related that Dollond met with a pot of uncommonly
pure flint glass in 1760, but even with this advantage of material he
admitted that, after numerous attempts, he could not provide really
excellent object-glasses of more than 3-3/4-inches diameter. It may
therefore be readily imagined that a refractor of 4½ or 5-inches
aperture was an instrument of great rarity and expense. Towards the
latter part of the 18th century Tulley’s price was £275 for a 5-inch
equatoreally mounted.
[Illustration: Fig. 9.
10-inch Reflecting-Telescope on a German Equatoreal, by Calver.]
In later years marked improvements were effected in the manufacture of
glass. A sign of this is apparent in the fact that, in 1829, Sir James
South was enabled to purchase a 12-inch lens. Four years before this
the Dorpat telescope, having an objective of 9½ inches, had created
quite a sensation. As time went on, still larger glasses were made. In
1862 Alvan Clark & Sons, of New York, U.S.A., finished an instrument
of 18½-inches aperture, at a cost of £3700; and in 1869 Cooke & Sons
mounted a 24·6-inch object-glass for the late Mr. Newall, of Gateshead.
The latter instrument was much larger than any other refractor hitherto
made, but it was not long to maintain supremacy. One of 25·8 inches
and 29-feet focus was finished in 1872 by Alvan Clark & Sons for the
Naval Observatory, Washington, at a cost of £9000. Another, of similar
size, was supplied by the same firm to Mr. McCormick, U.S.A. Several
important discoveries, including the satellites of Mars, were effected
with the great Washington telescope. A few years later a 27-inch was
completed by Grubb for the Vienna Observatory, and quite recently the
four largest refractors ever made have been placed in position and
are actively employed in various departments of work. These include a
29-inch by Martin for the Paris Observatory, a 30-inch by Henry Bros.
for Nice, a 30-inch by A. Clark & Sons for Pulkowa, and a 36-inch,
also by A. Clark & Sons, for the Lick Observatory on Mount Hamilton
in California. The latter has no rival in point of size, though
rumours are current that still larger lenses are in contemplation.
The tube of the 36-inch is 56 feet long and 3½ feet in diameter at
the ends, but the diameter is greater in the middle. It is placed
within a great dome 75 feet in diameter. The expense of the entire
apparatus is given as follows:—Cost of the dome, $56,850; of the visual
objective, $53,000; of the photographic objective, $13,000; of the
mounting, $42,000. Total, $164,850. This noble instrument—due to the
munificence of one individual, the late Mr. James Lick, of Chicago, who
bequeathed $700,000 for the purpose—may be regarded as the king of
refracting-telescopes. Placed on the summit of Mount Hamilton, where
the atmosphere is exceptionally favourable for celestial observations,
and utilized as its resources are by some of the best observers in
America, we may confidently expect it to largely augment our knowledge
of the heavenly bodies.
The great development in the powers of both refracting and
reflecting-telescopes, as a means of astronomical discovery,
exemplifies in a remarkable degree the ever-increasing resources and
refinements of mechanical art. In 1610 Galilei, from his window at
Padua, first viewed the moon and planets with his crude instrument
having a power of 3, and he achieved much during the remaining years he
lived, by increasing it tenfold, so that at last he could magnify an
object 30 times. Huygens laboured well in the same field; and others
who succeeded him formed links in the chain of progress which has
almost uninterruptedly run through all the years separating Galilei’s
time from our own. The primitive efforts of the Florentine philosopher
appear to have had their sequel in the magnificent telescope which has
lately been erected under the pure sky of Mount Hamilton. The capacity
of this instrument relatively to that of earlier ones may be judged
from the fact that a power of about 3300 times has lately been employed
with success in the measurement of a close and difficult double star.
Could Galilei but stand for a few moments at the eyepiece of this great
refractor, and contemplate the same objects which he saw, nearly three
centuries ago, through his imperfect little glasses at Padua, he would
be appalled at the splendid achievements of modern science.
FOOTNOTES:
[1] Galileo Galilei is very generally called by his christian name, but
I depart from this practice here.
[2] ‘Observatory,’ vol. ii. p. 364.
[3] Reproduced, by permission, from Cassell’s ‘New Popular Educator.’
[4] Reproduced, by permission, from Cassell’s ‘New Popular Educator.’