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.