A more artistic instrument than a simple candle is a small concave
reflector pierced through its centre, such as is used by
physicians in examining the throat.

[Illustration with caption: IMAGE OF CANDLE-FLAME IN OBJECT-
GLASS.]

[Illustration with caption: TESTING ADJUSTMENT OF OBJECT-GLASS.]

Place this reflector in the prolongation of the optical axis, set
the candle so that the light from the reflector shall be shown
through the glass, and look through the opening. Images of the
reflector itself will then be seen in the object-glass, and if the
adjustment is perfect, the reflector can be moved so that they
will all come into coincidence together.

When the objective is in the tube of the telescope, it is always
well to examine this adjustment from time to time, holding the
candle so that its light shall shine through the opening
perpendicularly upon the object-glass. The observer looks upon one
side of the flame, and then upon the other, to see if the images
are symmetrical in the different positions. If in order to see
them in this way the candle has to be moved to one side of the
central line of the tube, the whole objective must be adjusted. If
two images coincide in one position of the candle-flame, and two
in another position, so that they cannot all be brought together
in any position, it shows that the glasses are not properly
adjusted in their cell. It may be remarked that this last
adjustment is the proper work of the optician, since it is so