Editorial note — ILLUSTRATIONS (continued 2)

[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.]