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.
[Footnote: 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.
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