sing a
cement which is plastic before it sets, the disc may be easily taken
off the chuck and centred by a little handicraft, i.e. by rotating
the lathe slowly and pushing the disc into such a position that it
rotates about its axis. The grinding of the second surface is
accomplished exactly as in the former case; of course on reversing
the glass the chuck has to be slightly turned up to fit the convex or
concave surface.

Sec. 58. There is, however, one point of interest and importance--attention
to which will save a good deal of useless labour afterwards.
The glass must be ground in such a manner that the thickness at the
edge is the same all round. In other words, the axes of figure of the
two surfaces must coincide. This will be the case if the recentering
has been accurately performed, and therefore no pains should be spared
to see that it is exactly carried out. Any simple form of vernier
gauge (such as Brown and Sharpe's vernier callipers) will serve to
allow of a sufficiently accurate measurement of the edge thickness of
the lens. If any difference of thickness is observed as the gauge
moves round the edge, one or other of the surfaces must be reground.
Of course the latitude of error which may be permitted depends so much
on the final arrangements for a special finishing process called the
"centering of the lens"--which will be described--that it is
difficult to fix a limit, but perhaps one-thousandth of an inch may be
mentioned as a suitable amount for a 2-inch disc. For rough work, of
course; more margin may be admitted.

Sec. 59. In a large shop I imagine that lenses of only two inches
diameter would be ground in nests; or, in other words, a number would
be worked at a time, and centering, even of a rough kind, would be
left to the last; but this process will be treated hereafter. At
present I shall assume that only one lens will be made at a time.
Consequently we now enter on the stage of fine grinding by hand. A
leaden pedestal, for the sake of stability, must be provided on which
to mount the lens, so that the surface to be operated on may be nearly
horizontal (Fig. 50). Before this can be done, however, fresh
grinding tools (two for each surface) must be properly prepared.
After trying several plans I unhesitatingly recommend that all
fine-grinding surfaces should be made of glass. This is easily done
by taking two discs of lead, or iron, or slate, cut to a one-tenth
inch smaller radi