wrong; but the trouble may lie either in the object glass, in the
eyepiece, in the eye of the observer himself, or in the adjustment of
the lenses in the tube. A careful examination of the image and the
out-of-focus circles will enable us to determine with which of these
sources of error we have to deal. If the star image when in focus has a
sort of wing on one side, and if the rings out of focus expand
eccentrically, appearing wider and larger on one side than on the other,
being at the same time brightest on the least expanded side, then the
object glass is probably not at right angles to the axis of the tube and
requires readjustment. That part of the object glass on the side where
the rings appear most expanded and faintest needs to be pushed slightly
inward. This can be effected by means of counterscrews placed for that
purpose in or around the cell. But it, after we have got the object
glass properly squared to the axis of the tube or the line of sight, the
image and the ring system in and out of focus still appear oblong, the
fault of astigmatism must exist either in the objective, the eyepiece,
or the eye. The chances are very great that it is the eye itself that is
at fault. We may be certain of this if we find, on turning the head so
as to look into the telescope with the eye in different positions, that
the oblong image turns with the head of the observer, keeping its major
axis continually in the same relative position with respect to the eye.
The remedy then is to consult an oculist and get a pair of cylindrical
eyeglasses. If the oblong image does not turn round with the eye, but
does turn when the eyepiece is twisted round, then the astigmatism is in
the latter. If, finally, it does not follow either the eye or the
eyepiece, it is the objective that is at fault.

But instead of being oblong, the image and the rings may be misshapen in
some other way. If they are three-cornered, it is probable that the
object glass is subjected to undue pressure in its cell. This, if the
telescope has been brought out on a cool night from a warm room, may
arise from the unequal contraction of the metal work and the glass as
they cool off. In fact, no good star image can be got while a telescope
is assuming the temperature of the surrounding atmosphere. Even the air
inclosed in the tube is capable of making much trouble until its
temperature has sunk to the level of that outside. Half an hour at least
is required for a tele