bent and cut so as to fit the tool rest, and project beneath the
glass. The iron must be fairly rigid, for if it springs appreciably
beneath the pressure of the glass, it will not grind the latter really
round. The lathe may run rather faster than for turning cast iron of
the same size. Coarse emery, passing through a sieve of 80 threads to
the inch (run), may be fed in between the glass and iron, and the
latter screwed up till the disc just grinds slightly as it goes round.

A beginner will generally (in this as in all cases of grinding
processes) tend to feed too fast--no grinding process can be hurried.
If a slide rest is not available, a hinged board, carrying a bit of
iron, may (see Fig. 45) be arranged so as to turn about its hinge at
the back of the lathe; and it may be screwed up readily enough by
passing a long set-screw through the front edge, so that the point of
the screw bears upon the lathe bed. I may add that emery behaves as
if it were greasy, and it is difficult to wet it with clean water.
This is easily got over by adding a little soap or alcohol to the
water, or exercising a little patience.

A good supply of emery and water should be kept between the disc and
the iron; a little putty may be arranged round the point of contact
on the iron to form a temporary trough. In any case the resulting
emery mud should on no account be thrown away, but should be carefully
kept for further use. The process is complete when the glass is
perfectly round and of the required diameter as tested by callipers.

Sec. 55. The next step is to rough out the lens, and this may easily be
done by rotating it more slowly, i.e. with a surface speed of ten
feet per minute, and turning the glass with a hard file, as explained
in Sec. 42. If it is desired to employ the slide rest, it is quicker and
better to use a diamond tool--an instrument quite readily made, and
of great service for turning emery wheels and the like,--a thing, in
fact, which no workshop should be without. A bit of diamond bort, or
even a clear though off-colour stone, may be employed.

An ordinary lathe tool is prepared by drawing down the tool steel to a
long cone, resembling the ordinary practice in preparing a boring
tool. The apex of the cone must be cut off till it is only slightly
larger than the greatest transverse diameter of the diamond splinter.
The latter may have almost any shape--a triangular point, one side of
a three-sided prism