as to secure perfect
vision through them is the most difficult branch of the art of the
optician, and upon his skill in practising it will depend more than
upon anything else his ultimate success and reputation. The shaping of
a pair of lenses in the way we have described is not beyond the power
of any person of ordinary mechanical ingenuity, possessing the
necessary delicacy of touch and appreciation of the problem he is
attacking. But to make a perfect objective of considerable size, which
shall satisfy all the wants of the astronomer, is an undertaking
requiring such accuracy of eyesight, and judgment in determining where
the error lies, and such skill in manipulating so as to remove the
defects, that the successful men in any one generation can be counted
on one's fingers.

In order that the telescope may finally perform satisfactorily it is
not sufficient that the lenses should both be of proper figure; they
must also both be properly centred in their cells. If either lens is
tipped aside, or slid out from its proper central line, the definition
will be injured. As this is liable to happen with almost any telescope,
we shall explain how the proper adjustment is to be made.

The easiest way to test this adjustment is to set the cell with the two
glasses of the objective in it against a wall at night, and going to a
short distance, observe the reflection in the glass of the flame of a
candle held in the hand. Three or four reflections will be seen from
the different surfaces. The observer, holding the candle before his
eye, and having his line of sight as close as possible to the flame,
must then move until the different images of the flame coincide with
each other. If he cannot bring them into coincidence, owing to
different pairs coinciding on different sides of the flame, the glasses
are not perfectly centred upon each other. When the centring is
perfect, the observer having the light in the line of the axes of the
lenses, and (if it were possible to do so) looking through the centre
of the flame, would see the three or four images all in coincidence. As
he cannot see through the flame itself, he must look first on one side
and then on the other, and see if the arrangement of the images seen in
the lenses is symmetrical. If, going to different distances, he finds
no deviation from symmetry, in this respect the adjustment is near
enough for all practical purposes.

A more artistic instrument than a simple candle