ed with regularity, a proper "mounting" is as
essential as a good instrument. Persons unpractised in the use of such
instruments are very apt to underrate the importance of those
accessories which merely enable us to point the telescope. An idea of
what is wanted in the mounting may readily be formed if the reader will
try to look at a star with an ordinary good-sized spy-glass held in the
hand, and then imagine the difficulties he meets with multiplied by
fifty.

The smaller and cheaper telescopes, as commonly sold, are mounted on a
simple little stand, on which the instrument admits of a horizontal and
vertical motion. If one only wants to get a few glimpses of a celestial
object, this mounting will answer his purpose. But to make anything
like a study of a celestial body, the mounting must be an equatorial
one; that is, one of the axes around which the telescope moves must be
inclined so as to point towards the pole of the heavens, which is near
the polar star. This axis will then make an angle with the horizon
equal to the latitude of the place. The telescope cannot, however, be
mounted directly on this axis, but must be attached to a second one,
itself fastened to this one.

[Illustration with caption: SECTION OF THE PRIMITIVE MOUNTING. P P.
Polar axis, bearing a fork at the upper end A. Declination axis passing
through the fork E. Section of telescope tube C. Weight to balance the
tube.]

When mounted in this way, an object can be followed in its diurnal
motion from east to west by turning on the polar axis alone. But if the
greatest facility in use is required, this motion must be performed by
clock-work. A telescope with this appendage will commonly cost one
thousand dollars and upward, so that it is not usually applied to very
small ones.

We will now suppose that the reader wishes to purchase a telescope or
an object-glass for himself, and to be able to judge of its
performance. He must have the object-glass properly adjusted in its
tube, and must use the highest power; that is, the smallest eye-piece,
which he intends to use in the instrument. Of course he understands
that in looking directly at a star or a celestial object it must appear
sharp in outline and well defined. But without long practice with good
instruments, this will not give him a very definite idea. If the person
who selects the telescope is quite unpractised, it is possible that he
can make the best test by ascertaining at what distance