ried 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 assemb