The New Heavens | Page 6

magnitude. Herschel's 18-inch
reflector, with an area 5,000 times as great, rendered visible stars of the
fifteenth magnitude. The 60-inch reflector, with an area 57,600 times
that of the eye, reveals stars of the eighteenth magnitude, while to reach
stars of about the twentieth magnitude, photographic exposures of four
or five hours suffice with this instrument.
Every gain of a magnitude means a great gain in the number of stars
rendered visible. Stars of the second magnitude are 3.4 times as
numerous as those of the first, those of the eighth magnitude are three
times as numerous as those of the seventh, while the sixteenth
magnitude stars are only 1.7 as numerous as those of the fifteenth
magnitude. This steadily decreasing ratio is probably due to an actual
thinning out of the stars toward the boundaries of the stellar universe,

as the most exhaustive tests have failed to give any evidence of
absorption of light in its passage through space. But in spite of this
decrease, the gain of a single additional magnitude may mean the
addition of many millions of stars to the total of those already shown
by the 60-inch reflector. Here is one of the chief sources of interest in
the possibilities of a 100-inch reflecting telescope.
100-INCH TELESCOPE
[Illustration: Fig. 10. One-hundred-inch mirror, just silvered, rising out
of the silvering-room in pier before attachment to lower end of
telescope tube. (Seen above.)]
In 1906 the late John D. Hooker, of Los Angeles, gave the Carnegie
Institution of Washington a sum sufficient to construct a telescope
mirror 100 inches in diameter, and thus large enough to collect 160,000
times the light received by the eye. (Fig. 10.) The casting and annealing
of a suitable glass disk, 101 inches in diameter and 13 inches thick,
weighing four and one-half tons, was a most difficult operation, finally
accomplished by a great French glass company at their factory in the
Forest of St. Gobain. A special optical laboratory was erected at the
Pasadena headquarters of the Mount Wilson Observatory, and here the
long task of grinding, figuring, and testing the mirror was successfully
carried out by the observatory opticians. This operation, which is one
of great delicacy, required years for its completion. Meanwhile the
building, dome, and mounting for the telescope were designed by
members of the observatory staff, and the working drawings were
prepared. An opportune addition by Mr. Carnegie to the endowment of
the Carnegie Institution of Washington, of which the observatory is a
branch, permitted the necessary appropriations to be made for the
completion and erection of the telescope. Though delayed by the war,
during which the mechanical and optical facilities of the observatory
shops were utilized for military and naval purposes, the telescope is
now in regular use on Mount Wilson.
The instrument is mounted on a massive pier of reinforced concrete, 33
feet high and 52 feet in diameter at the top. A solid wall extends south
from this pier a distance of 50 feet, on the west side of which a very

powerful spectrograph, for photographing the spectra of the brightest
stars, will be mounted. Within the pier are a photographic dark room, a
room for silvering the large mirror (which can be lowered into the pier),
and the clock-room, where stands the powerful driving-clock, with
which the telescope is caused to follow the apparent motion of the stars.
(Fig. 11.)
[Illustration: Fig. 11. The driving-clock and worm-gear that cause the
100-inch Hooker telescope to follow the stars.]
The telescope mounting is of the English type, in which the telescope
tube is supported by the declination trunnions between the arms of the
polar axis, built in the form of a rectangular yoke carried by bearings
on massive pedestals to the north and south. These bearings must be
aligned exactly parallel to the axis of the earth, and must support the
polar axis so freely that it can be rotated with perfect precision by the
driving-clock, which turns a worm-wheel 17 feet in diameter, clamped
to the lower end of the axis. As this motion must be sufficiently
uniform to counteract exactly the rotation of the earth on its axis, and
thus to maintain the star images accurately in position in the field of
view, the greatest care had to be taken in the construction of the
driving-clock and in the spacing and cutting of the teeth in the large
worm-wheel. Here, as in the case of all of the more refined parts of the
instrument, the work was done by skilled machinists in the observatory
shops in Pasadena or on Mount Wilson after the assembling of the
telescope. The massive sections of the instrument, some of which
weigh as much as ten tons each, were constructed at Quincy, Mass.,
where machinery sufficiently large to build battleships