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 difficult
that the user of the telescope cannot ordinarily effect it. But the
perpendicularity of the whole objective to the tube of the telescope is
liable to be deranged in use, and every one who uses such an instrument
should be able to rectify an error of this kind.

The question may be asked, How much of a telescope can an amateur
observer, under any circumstances, make for himself? As a general rule,
his work in this direction must be confined to the tube and the
mounting. We should not, it is true, dare to assert that any ingenious
young man, with a clear appreciation of optical principles, could not
soon learn to grind and polish an object-glass for himself by the
method we have described, and thus obtain a much better instrument than
Galileo ever had at his command. But it would be a wonderful success if
his home-made telescope was equal to the most indifferent one which can
be bought at an optician's. The objective, complete in itself, can be
purchased at prices depending upon the size.

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