od 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.

A star of the first magnitude should next be brought into the field of
view. If an irradiation from one side is perceived, part of the
object-glass has not the same refractive p