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.
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