in, will be formed of a great number of minute rings of light. If
the glass is good, these rings will be round, unbroken, and
equally bright. We present several figures showing how these
spectral images, as they are sometimes called, will appear; first,
when the eye-piece is pushed in, and secondly, when it is drawn
out, with telescopes of different qualities.

We have thus far spoken only of the refracting telescope, because
it is the kind with which an observer would naturally seek to
supply himself. At the same time there is little doubt that the
construction of a reflector of moderate size is easier than that
of a corresponding refractor. The essential part of the reflector
is a slightly concave mirror of any metal which will bear a high
polish. This mirror may be ground and polished in the same way as
a lens, only the tool must be convex.

[Illustration with caption: SPECTRAL IMAGES OF STARS; THE UPPER
LINE SHOWING HOW THEY APPEAR WITH THE EYE-PIECE PUSHED IN, THE
LOWER WITH THE EYE-PIECE DRAWN OUT.

A The telescope is all right
B Spherical aberration shown by the light and dark centre
C The objective is not spherical but elliptical
D The glass not uniform--a very bad and incurable case
E One side of the objective nearer than the other. Adjust it]

Of late years it has become very common to make the mirror of
glass and to cover the reflecting face with an exceedingly thin
film of silver, which can be polished by hand in a few minutes.
Such a mirror differs from our ordinary looking-glass in that the