The telescope should be well balanced for vision with the small astronomical eye-pieces.  But as there is often occasion to use appliances which disturb the balance, it is well to have the means of at once restoring equilibrium.  A cord ring running round the tube (pretty tightly, so as to rest still when the tube is inclined), and bearing a small weight, will be all that is required for this purpose; it must be slipped along the tube until the tube is found to be perfectly balanced.  Nothing is more annoying than, after getting a star well in the field, to see the tube shift its position through defective balance, and thus to have to search again for the star.  Even with such an arrangement as is shown in fig. 8, though the tube cannot readily shift its position, it is better to have it well balanced.

The quality of the stand has a very important influence on the performance of a telescope.  In fact, a moderately good telescope, mounted on a steady stand, working easily and conveniently, will not only enable the observer to pass his time much more pleasantly, but will absolutely exhibit more difficult objects than a finer instrument on a rickety, ill-arranged stand.  A good observing-chair is also a matter of some importance, the least constraint or awkwardness of position detracting considerably from the power of distinct vision.  Such, at least, is my own experience.

But the mere examination of the glasses, tube, mounting, &c., is only the first step in the series of tests which should be applied to a telescope, since the excellence of the instrument depends, not on its size, the beauty of its mounting, or any extraneous circumstances, but on its performance.

The observer should first determine whether the chromatic aberration is corrected.  To ascertain this the telescope should be directed to the moon, or (better) to Jupiter, and accurately focussed for distinct vision.  If, then, on moving the eye-piece towards the object-glass, a ring of purple appears round the margin of the object, and on moving the eye-glass in the contrary direction a ring of green, the chromatic aberration is corrected, since these are the colours of the secondary spectrum.

To determine whether the spherical aberration is corrected, the telescope should be directed towards a star of the third or fourth magnitude, and focussed for distinct vision.  A cap with an aperture of about one-half its diameter should then be placed over the object-glass.  If no new adjustment is required for distinct vision, the spherical aberration is corrected, since the mean focal length and the focal length of the central rays are equal.  If, when the cap is on, the eye-piece has to be pulled out for distinct vision, the spherical aberration has not been fully corrected; if the eye-piece has to be pushed in, the aberration has been over-corrected.  As a further test, we may cut off the central rays, by means of a circular card covering the middle of the object-glass, and compare the focal length for distinct vision with the focal length when the cap is applied.  The extent of the spherical aberration may be thus determined; but if the first experiment gives a satisfactory result, no other is required.