ent.

The discs are then dressed circular on a grindstone, the rod serving
both as a gauge and handle. A sufficient number of these discs having
been prepared, a pair of brass tools of the form shown in the sketch
(Fig. 55), and of about the proper radius of curvature, are made.
One of these tools is used as a support for the glass discs.

Fig. 55.

A compass being set to scribe circles of the same diameter as the
glass discs, centre marks are made on the surface of the appropriate
tool, circles are drawn on this, and facets are filed or milled (for
which the spiral head of the milling machine is excellent). In the
case of concave supporting surfaces, i.e. in making concave lenses, I
apprehend filing would be difficult, and the facets would have to be
made by a rose cutter or mill; but if the discs are fairly round,
then, in fact, no facets are required.

The facets being ready, the glass discs are cemented to them by
centering cement, which may be used quite generally for small lenses.
When the cutting of facets has been omitted on a concave surface, the
best cement is hard pitch. The grinding tool is generally rather
larger than the nest of lenses. Coarse and fine grinding is
accomplished wholly on the lathe--the tool being rotated at a fair
speed (see infra), and the nest of lenses moved about by its handle so
as to grind all parts equally. It must, of course, be held anywhere
except "dead on," for then the part round the axis would not get
ground; this inoperative portion of the rotating tool must therefore
be allowed to distribute its incapable efforts evenly over the nest of
lenses.

Polishing is accomplished by means of the grinding tool, coated with
paper and rouge as before; or the tool may be coated with very thin
cloth and used with rouge as before--in this case the polishing goes
on fastest when the surface of the cloth is distinctly damp. In
working by this method, each grade of emery need only be applied from
five to ten minutes. The glass does not appear to get scratched when
the emery is changed, provided everything is well washed. A good
polish may be got in an hour. The lathe is run as for turning brass
of the same diameter as the tool.

One side of the lenses being thus prepared, they are reversed, and the
process gone through for the other side in a precisely similar manner.
[Footnote: Unless the radius of curvature is very short and the lenses
also convex, there is no necessity