Still, it is well for the observer who wishes systematically to survey the heavens—­and who can afford the expense—­to obtain a well-mounted equatorial.  In this method of mounting, the main axis is 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 small telescope.  It can be attached by wires to the telescope-tube, and adjusted each night before commencing observation.  The adjustment is thus managed:—­a low power being applied to the telescope, the tube is turned towards a bright star;