observer stands on a platform,
which is moved up and down by electric motors in front of the opening
in the dome through which the observations are made.

[Illustration: Fig. 14. Spiral nebula in Andromeda, seen edge on
(Ritchey).

Photographed with the 60-inch telescope.]

Other arrangements of the telescope, for which auxiliary convex
mirrors carried near the upper end of the tube are required, permit
the image to be photographed at the side of the tube near its lower
end, either with or without a spectrograph; or with a very powerful
spectrograph mounted within a constant-temperature chamber south
of the telescope pier. In this last case, the light of a star is
so reflected by auxiliary mirrors that it passes down through a
hole in the south end of the polar axis and brings the star to
a focus on the slit of the fixed spectrograph.

ATMOSPHERIC LIMITATIONS

The huge dimensions of such a powerful engine of research as the
Hooker telescope are not in themselves a source of satisfaction to
the astronomer, for they involve a decided increase in the labor
of observation and entail very heavy expense, justifiable only in
case important results, beyond the reach of other instruments,
can be secured. The construction of a telescope of these dimensions
was necessarily an experiment, for it was by no means certain, after
the optical and mechanical difficulties had been overcome, that
even the favorable atmosphere of California would be sufficiently
tranquil to permit sharply defined celestial images to be obtained
with so large an aperture. It is therefore important to learn what
the telescope will actually accomplish under customary observing
conditions.

Fortunately we are able to measure the performance of the instrument
with certainty. Close beside it on Mount Wilson stands the 60-inch
reflector, of similar type, erected in 1908. The two telescopes can
thus be rigorously compared under identical atmospheric conditions.

The large mirror of the 100-inch telescope has an area about 2.8
times that of the 60-inch, and therefore receives nearly three times
as much light from a star. Under atmospheric conditions perfect
enough to allow all of this light to be concentrated in a point,
it should be capable of recording on a photographic plate, with a
given exposure, stars about one magnitude fainter than the faintest
stars within reach of the 60-inch. The increased focal length,
permitting such objects as the moon to be