ting the glass
discs out of glass sheet, which also may be purchased of suit-able
glass. To do this, the simplest way is to begin by cutting squares
and then cutting off the corners with the diamond, the approximate
circular figure being obtained by grinding the edges on an ordinary
grindstone.

If the pieces are larger, time and material may be saved by using a
diamond compass, i.e. an ordinary drawing compass armed with a
diamond to cut circles on the glass, and breaking the superfluous
glass away by means of a pair of spectacle-maker's shanks (Fig. 44),
or what does equally well, a pair of pliers with soft iron jaws. With
these instruments glass can be chipped gradually up to any line,
whether diamond-cut or not, the jaws of the pincers being worked
against the edge of the glass, so as to gradually crush it away.

Fig. 44.

Assuming that the glass has been bought or made roughly circular, it
must be finished on the lathe. For this purpose it is necessary to
chuck it on an iron or hardwood chuck, as shown in Fig. 46. For a
lens below say an inch in diameter, the centering cement may be used;
but for a lens of a diameter greater than this, sufficient adhesion is
easily obtained with Regnault's mastic, and its low melting point
gives it a decided advantage over the shellac composition.

The glass may be heated gradually by placing it on the water bath, or
actually in the water, and gradually bringing the water up to the
boiling-point. The glass, being taken out, is rapidly wiped, and
rubbed with a bit of waste moistened, not wet, with a little
turpentine: its surface is then rubbed with a stick of mastic
previously warmed so as to melt easily. The surface of the chuck
being also warm, and covered with a layer of melted cement, it is
applied to the glass. The lathe is turned slowly by hand, and the
glass pushed gradually into the most central position; it is then
pressed tight against the chuck by the back rest, a bit of wood being
interposed for obvious reasons.

When all is cold the turning may be proceeded with. The quickest way
is to use the method already described (i.e. actual turning by a file
tool); but if the student prefers (time being no object), he may
accomplish the reduction to a circular form very easily by grinding.

Fig. 45.

Fig. 46.

For this purpose he will require to make the following arrangements
(Fig. 45). If the lathe has a slide rest, a piece of stout iron may
be