Tin, if it be of sufficient thickness, would be a very good
material. The brighter it is kept, the better. The work of fitting
the objective into one end of a tin tube of double thickness, and
properly adjusting it, will probably be quite within the powers of
the ordinary amateur. The fitting of the eye-piece into the other
end of the tube will require some skill and care both on his own
part and that of his tinsmith.
Although the construction of the eye-piece is much easier than
that of the objective, since the same accuracy in adjusting the
curves is not necessary, yet the price is lower in a yet greater
degree, so that the amateur will find it better to buy than to
make his eye-piece, unless he is anxious to test his mechanical
powers. For a telescope which has no micrometer, the Huyghenian or
negative eye-piece, as it is commonly called, is the best. As made
by Huyghens, it consists of two plano-convex lenses, with their
plane sides next the eye, as shown in the figure.
[Illustration with caption: THE HUYGHENIAN EYE-PIECE.]
So far as we have yet described our telescope it is optically
complete. If it could be used as a spy-glass by simply holding it
in the hand, and pointing at the object we wish to observe, there
would be little need of any very elaborate support.
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