s directed to
the pole of the heavens; the other axis, at right angles to the first,
carries the telescope-tube. One of the many methods adopted for mounting
equatorials is that exhibited--with the omission of some minor
details--in fig. 9. _a_ is the polar axis, _b_ is the axis (called the
declination axis) which bears the telescope. The circles _c_ and _d_
serve to indicate, by means of verniers revolving with the axes, the
motion of the telescope in right ascension and declination,
respectively. The weight _w_ serves to counterpoise the telescope, and
the screws _s_, _s_, _s_, _s_, serve to adjust the instrument so that
the polar axis shall be in its proper position. The advantage gained by
the equatorial method of mounting is that only one motion is required to
follow a star. Owing to the diurnal rotation of the earth, the stars
appear to move uniformly in circles parallel to the celestial equator;
and it is clear that a star so moving will be kept in the field of
view, if the telescope, once directed to the star, be made to revolve
uniformly and at a proper rate round the polar axis.

[Illustration: _Fig. 9._]

The equatorial can be directed by means of the circles _c_ and _d_ to
any celestial object whose right ascension and declination are known. On
the other hand, to bring an object into the field of view of an
alt-azimuth, it is necessary, either that the object itself should be
visible to the naked eye, or else that the position of the object should
be pretty accurately learned from star-maps, so that it may be picked up
by the alt-azimuth after a little searching. A small telescope called a
_finder_ is usually attached to all powerful telescopes intended for
general observation. The finder has a large field of view, and is
adjusted so as to have its axis parallel to that of the large telescope.
Thus a star brought to the centre of the large field of the finder
(indicated by the intersection of two lines placed at the focus of the
eye-glass) is at, or very near, the centre of the small field of the
large telescope.

If a telescope has no finder, it will be easy for the student to
construct one for himself, and will be a useful exercise in optics. Two
convex lenses not very different in size from those shown in fig. 1, and
placed as there shown--the distance between them being the sum of the
focal lengths of the two glasses--in a small tube of card, wood, or tin,
will serve the purpose of a finder for a smal