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 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.
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