The New Heavens | Page 5

or as subordinate to the stellar system
which includes our minute group of sun and planets, the great star
clouds of the Milky Way, and the distant globular star clusters.
[Illustration: Fig. 7. Section of a steel girder for dome covering the
100-inch telescope, on its way up Mount Wilson.]
These few particulars may give a slight conception of the scale of the
known universe, but a word must be added regarding some of its most
striking phenomena. The great majority of the stars whose motions
have been determined belong to one or the other of two great star
streams, but the part played by these streams in the sidereal system as a
whole is still obscure. The stars have been grouped in classes,
presumably in the order of their evolutional development, as they pass
from the early state of gaseous masses, of low density, through the
successive stages resulting from loss of heat by radiation and increased
density due to shrinkage. Strangely enough, their velocities in space
show a corresponding change, increasing as they grow older or perhaps
depending upon their mass.
It is impossible within these limits to do more than to give some
indication of the scope of the new astronomy. Enough has been said,
however, to assist in appreciating the increased opportunity for
investigation, and the nature of the heavy demands made upon the
modern observatory. But before passing on to describe one of the latest

additions to the astronomer's instrumental equipment, a word should be
added regarding the chief classes of telescopes.
REFRACTORS AND REFLECTORS
Astronomical telescopes are of two types: refractors and reflectors. A
refracting telescope consists of an object-glass composed of two or
more lenses, mounted at the upper end of a tube, which is pointed at the
celestial object. The light, after passing through the lenses, is brought to
a focus at the lower end of the tube, where the image is examined
visually with an eyepiece, or photographed upon a sensitive plate. The
largest instruments of this type are the 36-inch Lick telescope and the
40-inch refractor of the Yerkes Observatory.
[Illustration: Fig. 8. Erecting the steel building and revolving dome that
cover the Hooker telescope.]
Reflecting telescopes, which are particularly adapted for photographic
work, though also excellent for visual observations, are very differently
constructed. No lens is used. The telescope tube is usually built in
skeleton form, open at its upper end, and with a large concave mirror
supported at its base. This mirror serves in place of a lens. Its upper
surface is paraboloidal in shape, as a spherical surface will not unite in
a sharp focus the rays coming from a distant object. The light passes
through no glass--a great advantage, especially for photography, as the
absorption in lenses cuts out much of the blue and violet light, to which
photographic plates are most sensitive. The reflection occurs on the
upper surface of the mirror, which is covered with a coat of pure silver,
renewed several times a year and always kept highly burnished.
Silvered glass is better than metals or other substances for telescope
mirrors, chiefly because of the perfection with which glass can be
ground and polished, and the ease of renewing its silvered surface when
tarnished.
The great reflectors of Herschel and Lord Rosse, which were provided
with mirrors of speculum metal, were far inferior to much smaller
telescopes of the present day. With these instruments the star images
were watched as they were carried through the field of view by the

earth's rotation, or kept roughly in place by moving the telescope with
ropes or chains. Photographic plates, which reveal invisible stars and
nebulæ when exposed for hours in modern instruments, were not then
available. In any case they could not have been used, in the absence of
the perfect mechanism required to keep the star images accurately fixed
in place upon the sensitive film.
[Illustration: Fig. 9. Building and revolving dome, 100 feet in diameter,
covering the 100-inch Hooker telescope.
Photographed from the summit of the 150-foot-tower telescope.]
It would be interesting to trace the long contest for supremacy between
refracting and reflecting telescopes, each of which, at certain stages in
its development, appeared to be unrivalled. In modern observatories
both types are used, each for the purpose for which it is best adapted.
For the photography of nebulæ and the study of the fainter stars, the
reflector has special advantages, illustrated by the work of such
instruments as the Crossley and Mills reflectors of the Lick
Observatory; the great 72-inch reflector, recently brought into effective
service at the Dominion Observatory in Canada; and the 60-inch and
100-inch reflectors of the Mount Wilson Observatory.
The unaided eye, with an available area of one-twentieth of a square
inch, permits us to see stars of the sixth