ng 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 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 Li