light as if it
were as large as the lens O B. The rays, however, are convergent,
and the point difficult to [Page 44] find. Hence there is placed at
R a concave lens, passing through which the rays emerge in parallel
lines, and are received by the eye. Opera-glasses are made upon
precisely this principle to-day, because they can be made
conveniently short.

[Illustration: Fig. 13.--Refracting Telescope.]

If, instead of a concave lens at R, converting the converging rays
into parallel ones, we place a convex or magnifying lens, the minute
image is enlarged as much as an object seems diminished when the
telescope is reversed. This is the grand principle of the refracting
telescope. Difficulties innumerable arise as we attempt to enlarge
the instruments. These have been overcome, one after another, until
it is now felt that the best modern telescope, with an object lens
of twenty-six inches, has fully reached the limit of optical power.

_The Reflecting Telescope_.

This is the only kind of instrument differing radically from the
refracting one already described. It receives the light in a concave
mirror, M (Fig. 14), which reflects it to the focus F, producing the
same result as the lens of the refracting telescope. Here a mirror
may be placed obliquely, reflecting the image at right angles to the
eye, outside the tube, in which case it is called the Newtonian
telescope; or a mirror at R may be placed perpendicularly, and send
the rays through [Page 45] an opening in the mirror at M. This form
is called the Gregorian telescope. Or the mirror M may be slightly
inclined to the coming rays, so as to bring the point F entirely
outside the tube, in which case it is called the Herschelian
telescope. In either case the image may be magnified, as in the
refracting telescope.

[Illustration: Fig. 14.--Reflecting Telescope.]

Reflecting telescopes are made of all sizes, up to the Cyclopean
eye of the one constructed by Lord Rosse, which is six feet in
diameter. The form of instrument to be preferred depends on the
use to which it is to be put. The loss of light in passing through
glass lenses is about two-tenths. The loss by reflection is often
one-half. In view of this peculiarity and many others, it is held
that a twenty-six-inch refractor is fully equal to any six-foot
reflector.

The mounting of large telescopes demands the highest engineering
ability. The whole instrument, with its vast weight of a twenty-six-inch