Photographic plates, which reveal invisible
stars and nebulae 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 nebulae 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 magnitude.
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