William F. Denning.

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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.’




CHAPTER II.

_RELATIVE MERITS OF LARGE AND SMALL TELESCOPES._


The number of large telescopes having so greatly increased in recent
years, and there being every prospect that the demand for such
instruments will continue, it may be well to consider their advantages
as compared with those of much inferior size. Object-glasses and
specula will probably soon be made of a diameter not hitherto attained;
for it is palpably one of the ambitions of the age to surpass all
previous efforts in the way of telescopic construction. There are some
who doubt that such enormous instruments are really necessary, and
question whether the results obtained with them are sufficient return
for the great expense involved in their erection. Large instruments
require large observatories; and the latter must be at some distance
from a town, and in a locality where the atmosphere is favourable.
Nothing can be done with great aperture in the presence of smoke and
other vapours, which, as they cross the field, become ruinous to
definition. Moreover, a big instrument is not to be manipulated with
the same facility as a small one: and when anything goes wrong with
it, its rectification may be a serious matter, owing to the size.
Such telescopes need constant attention if they would be kept in
thorough working order. On the other hand, small instruments involve
little outlay, they are very portable, and require little space. They
may be employed in or out of doors, according to the inclination and
convenience of the observer. They are controlled with the greatest
ease, and seldom get out of adjustment. They are less susceptible to
atmospheric influences than larger instruments, and hence may be used
more frequently with success and at places by no means favourably
situated in this respect. Finally, their defining powers are of



Online LibraryWilliam F. DenningTelescopic Work for Starlight Evenings → online text (page 2 of 32)