It is impossible within these limits to do more than to give some
indication of the scope of the new astronomy. Enough has been said,
however, to assist in appreciating the increased opportunity for
investigation, and the nature of the heavy demands made upon the
modern observatory. But before passing on to describe one of the
latest additions to the astronomer's instrumental equipment, a
word should be added regarding the chief classes of telescopes.
REFRACTORS AND REFLECTORS
Astronomical telescopes are of two types: refractors and reflectors.
A refracting telescope consists of an object-glass composed of
two or more lenses, mounted at the upper end of a tube, which is
pointed at the celestial object. The light, after passing through
the lenses, is brought to a focus at the lower end of the tube, where
the image is examined visually with an eyepiece, or photographed
upon a sensitive plate. The largest instruments of this type are
the 36-inch Lick telescope and the 40-inch refractor of the Yerkes
Observatory.
[Illustration: Fig. 8. Erecting the steel building 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.
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