telescope to the heavens. He cannot be said to have invented the
telescope, but he certainly constructed his own without a pattern,
and used it to good purpose. It consists of a lens, O B (Fig. 13),
which acts as a multiple prism to bend all the rays to one point
at R. Place the eye there, and it receives as much 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