tion which are to be carried out. For this purpose it seems
desirable to begin by roughly indicating the various steps which are
to be taken.

(1) The glass is to be made circular in form and of a given diameter.

(2) Called Rough Grinding. The surfaces of the glass are to be made
roughly convex, plane, or concave, as may be required; the glass is to
be equally thick all round the edge. In this process the glass is
abraded by the use of sand or emery rubbed over it by properly shaped
pieces of iron or lead called "tools."

(3) The glass is ground with emery to the correct spherical figure as
given by a spherometer.

(4) Called Fine Grinding. The state of the surface is gradually
improved by grinding with finer and finer grades of emery.

(5) The glass is polished by rouge.

(6) The glass is "figured." This means that it is gradually altered in
form by a polishing tool till it gives the best results as found by
trial.

In processes 2 to 5 counterpart tool surfaces are required--as a rule
two convex and two concave surfaces for each lens surface. These
subsidiary surfaces are worked (i.e. ground) on discs of cast iron
faced with glass, or on slate discs; and discs thus prepared are
called "tools."

Taking these processes in the order named, the mode of manufacture is
shortly as follows:-

(1) The disc of glass, obtained in a roughly circular form, is mounted
on an ordinary lathe, being conveniently cemented by Regnault's mastic
to a small face plate. The lathe is rotated slowly, and the glass is
gradually turned down to a circular figure by means (1) of a tool with
a diamond point; or (2) an ordinary hand-file moistened with
kerosene, as described in Sec. 42; or (3) a mass of brass or iron served
with a mixture of emery--or sand--and water fed on to the disc, so
that the disc is gradually ground circular.

The operation of making a circular disc of given diameter does not
differ in any important particular from the similar operation in the
case of brass or iron, and is in fact merely a matter of turning at a
slow speed.

(2 and 3) Roughing or bringing the surfaces of the glass roughly to
the proper convex or concave shape. This is accomplished by
grinding, generally with sand in large works, or with emery in the
laboratory, where the time saved is of more importance than the value
of the emery.

Discs of iron or brass are cast and turned so as to have a diameter
slightly less than that of the gla