us of curvature (in the case of a convex tool, and
the opposite in the other case) than the lens surface (Fig. 51, A).
On these, square bits of sheet glass, one-tenth of an inch thick, are
to be cemented, so as to leave channels of about one-eighth of an inch
between each bit of glass (Fig. 52, B). The "mastic" cement
formerly described may be employed for this purpose.

Fig. 50.

The bits of glass ought first to have their edges dressed smooth on
the grind-stone. A convex and concave glass surface having been thus
roughly prepared, they must be mounted in turn in the lathe, and
brought to the proper curvature by grinding with the tools formerly
employed and tested by the template or spherometer. It is well to
control this process by means of a spherometer, so that the desired
radius may be approximately reached. The two glass-grinding tools
are then ground together by hand (see Sec. 53 and Sec. 61), the spherometer
being employed from time to time to check the progress of the work.
In general, if large circular sweeps are taken, greatly overhanging
the side of the glass surface to be figured, both the upper and lower
surfaces will be more ground at the edges, while in the opposite
event the centre will be chiefly affected.

Fig. 51.

A spherometer capable of measuring a 2-inch surface may be procured,
having a screw of, say, 50 threads to the inch, and a micrometer
surface divided into 200 parts, each part easily capable of
subdivision--into tenths or even twentieths. To get the full
advantage of the spherometer it must screw exceedingly freely (i.e.
must be well oiled with clock oil), and must not be fingered except at
the milled head. If one of the legs is held by the fingers the
expansion is sufficient to throw the instrument quite out of
adjustment. The glass-grinding tools being brought to the proper
figure, the next process is to transfer the same to the lens, and this
is done by similar means, the fellow tool being used to correct the
one employed in grinding the lens surface. Before the grade of emery
is changed all three surfaces must agree, as nearly, at least, as the
spherometer will show.

In order to prevent confusion the following summary of the steps
already taken may be given. The discs of glass are first ground or
turned so as to be truly circular. Four "tools" are made for each
surface--a rough pair of iron or lead, and a finishing pair of iron,
lead, or slate faced by glass s