The following is a rough rule for getting an idea of the
price of an achromatic objective, made to order, of the finest quality.
Take the cube of the diameter in inches, or, which is the same thing,
calculate the contents of a cubical box which would hold a sphere of
the same diameter as the clear aperture of the glass. The price of the
glass will then range from $1 to $1.75 for each cubic inch in this box.
For example, the price of a four-inch objective will probably range
from $64 to $112. Very small object-glasses of one or two inches may be
a little higher than would be given by this rule. Instruments which are
not first-class, but will answer most of the purposes of the amateur,
are much cheaper.]

[Illustration with caption: A VERY PRIMITIVE MOUNTING FOR A TELESCOPE.]

The tube for the telescope may be made of paper, by pasting a great
number of thicknesses around a long wooden cylinder. A yet better tube
is made of a simple wooden box. The best material, however, is metal,
because wood and pasteboard are liable both to get out of shape, and to
swell under the influence of moisture. 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. But if a telescope, even of the
smallest size, is to be us