en the eye-piece is pushed 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 coating of silver is put on
the front surface, so that the light does not pass through the glass.
Moreover, the coating of silver is so thin as to be almost transparent:
in fact, the sun may be seen through it by direct vision as a faint
blue object. Silvered glass reflectors made in this way are extensively
manufactured in London, and are far cheaper than refracting telescopes
of corresponding size. Their great drawback is the want of permanence
in the silver film. In the city the film will ordinarily tarnish in a
few months from the sulphurous vapors arising from gaslights and other
sources, and even in the country it is very difficult to preserve the
mirror from the contact of everything that will injure it. In
consequence, the possessor of such a telescope, if he wishes to keep it
in order, must always be prepared to resilver and repolish it. To do
this requires such careful manipulati