The progress in astronomical science throughout the nineteenth century
has naturally to a great extent depended upon the advances made both
in the optics of the telescope and the way in which they are mounted,
either with circles to record exact times and positions, or made to
move so as to keep a star or other celestial objects in the field of
view while under observation. The perfection of definition and the
magnitude of the lenses employed in the modern instrument have been
responsible for many important discoveries.
Ever since the telescope was invented—Galileo’s lens was smaller
than those used in spectacles—men’s minds have been concentrated on
producing instruments of larger and larger size to fathom the cosmos to
its innermost depths.
At the beginning of the century we were, as we have seen already, in
possession of reflectors of large dimensions; Herschel’s four-foot
mirror, the instrument he was using in 1801, which had a focal length
of forty feet, was capable of being employed with high magnifying
powers; and it was the judicious use of these, on occasions when the
finest of weather prevailed, that enabled him to enrich so extensively
our knowledge of the stellar and planetary systems. For the ordinary
work of astronomy, however, especially when circles are used,
refractors are the more suitable instruments. This form suffers less
from the vicissitudes of weather and temperature, and is, therefore,
more suited where exact measurements are required.
Towards the end of the eighteenth century a Swiss artisan, Pierre
Guinard, after many years of patient labor, succeeded in producing pure
disks of flint glass as large as six inches in diameter. The modern
refracting telescope thus became possible.
In 1804 there was started at Munich the famous optical and mechanical
institute, which soon made its presence felt in the astronomical world.
Reforms in instrument making were soon taken in hand, and under the
leadership of the great German astronomer, Bessel, great strides were
made in instruments of precision. Fraunhofer, who had been silently
working away at the theory of lenses, and making various experiments
in the manufacture of glass, was joined, in 1805, by Guinard. In 1809
Troughton invented a new method of graduating circles, according to
Airy the greatest improvement ever achieved in the art of instrument
making.
In 1824 Fraunhofer successfully completed and perfected an object-glass
of 9.9 inches in diameter for the Dorpat Observatory. This objective
might literally have been called a “giant,” for nothing approaching it
in size had been previously made.
England, which was at one time the exclusive seat of the manufacture of
refracting telescopes, was now completely outstripped by both Germany
and France, and for this we had to thank “the short-sighted policy of
the government, which had placed an exorbitant duty on the manufacture
of flint glass.” In 1833 the Dorpat refractor was eclipsed by one of
fifteen inches aperture made for the Pulkowa Observatory by Merz &
Mähler, Fraunhofer’s successors, who about ten years later supplied a
similar instrument to Harvard College. At that time Lord Rosse emulated
with success the efforts of Herschel and rehabilitated the reflector by
producing a metallic mirror of six-foot aperture and fifty-four-foot
focal length which he mounted at Parsonstown. The speculum weighed
no less than four tons. To mount this immense mass efficiently and
safely was a work of no light nature, but he successfully accomplished
it, and eventually both mirror and the telescope, which weighed now
altogether fourteen tons, were so well counterpoised that they could be
easily moved in a limited direction by means of a windlass worked by
two men. The perfection of the “seeing” qualities of this instrument
and its enormous light-grasping powers were particularly striking, and
observational astronomy was considerably enriched by the discoveries
made with it.
Speculum metal was not destined to stay; ten years later (1857) the
genius of Léon Foucault introduced glass mirrors with a thin coating of
silver deposited chemically, and these have now universally superseded
the metallic ones.
The long supremacy of Germany in the matter of refractors was broken
down ultimately by the famous English optician and engineer, Thomas
Cooke, of York. His first considerable instrument, one of seven
inches aperture, was finished in 1851; and in 1865, a year before his
lamented death, he completed the first of our present giant refractors,
one of twenty-five inches aperture, for Mr. Newall, of Gateshead.
In consequence of the success of Cooke’s achievement other large
refractors were soon undertaken.
Alvan Clarke, the famous optician of Cambridgeport, Massachusetts,
at once commenced a twenty-six-inch for the Washington Observatory.
The next was one of twenty-seven inches, made by Grubb for the Vienna
Observatory. Object-glasses now grew inch by inch in size, depending
on the increased dimensions of disks that could be satisfactorily cast.
Gautier, of Paris, completed a twenty-nine-and-a-half-inch for the Nice
Observatory, while Alvan Clarke made an objective of thirty inches for
Pulkowa. In 1877 the latter successfully completed the mounting of
an objective of thirty-six inches for the Lick Observatory, but this
immense lens was only achieved after a great number of failures. Even
this large object-glass was surpassed in size by the completion in 1892
of the forty-inch which he made for the Yerkes Observatory, and by that
made by Gautier for the Paris Exhibition of 1900.
So much, then, for the largest refractors. In recent years, since the
introduction of the silver on glass mirrors, with their stability of
figure and brilliant surface, which can be easily renewed, reflectors
of large apertures are again being produced. The first of these was
one of thirty-six inches aperture made by Calver for Dr. Common, who
demonstrated its fine qualities and his own skill by the beautiful
photographs of the nebula of Orion he was enabled to secure with
it. Dr. Common himself has since turned his attention to the making
and silvering of large mirrors of this kind, and the largest he has
actually completed and mounted equatorially is one with a diameter of
five feet. Another of thirty-six inches aperture is in use at the Solar
Physics Observatory at Kensington.
The progress of depositing silver on glass has led of late years to
important developments in which _plane_ mirrors are used. Foucault was
the first to utilize such mirrors in his “siderostat,” in which such a
mirror is made to move in front of a horizontal fixed telescope, which
may be of any focal length, and no expensive dome or rising floor is
required. The plane mirror of the siderostat in the Paris Exhibition
telescope is six feet in diameter.
A variation of this instrument is the cœlostat more recently advocated
by Lippmann. The Coudé equatorial mounting also depends upon the use of
plane mirrors; with such a telescope the observer is at rest at a fixed
eye-piece or camera in a room which may be kept at any temperature.
Now that in astronomical work eye observations are indispensably
supplemented by the employment of photography, an important
modification of the refracting telescope has become necessary; this was
first suggested by Rutherfurd.
The ordinary achromatic object-glass consists, as a rule, of two
lenses, one made of flint and the other of crown glass; but in this
form the photographic rays are not brought to the same focus as the
visual rays. This, however, can be achieved by employing three lenses
instead of two, each of different kinds of glass. The most modern
improvement in the telescope is due to Mr. Dennis Taylor, of Cooke &
Sons, and to Dr. Schott and Professor Abbe, whose researches in the
manufacture of old and new varieties of optical glass have rendered
Mr. Taylor’s results feasible. By the Taylor lens outstanding color is
abolished, all the rays being brought absolutely to the same focus;
such lenses can therefore be used either for visual observations or for
photography for spectroscopy.