is very convenient. A hole is drilled in the
steel (which must have been well softened), only just large enough to
allow the diamond to enter--if the splinter is thicker in the middle
than at either end, so much the better--the diamond is fastened in
position by squeezing the soft steel walls tightly down upon it.
Personally I prefer to use a tool holder, and in this case generally
mount the diamond in a bit of brass rod of the proper diameter; and
instead of pinching in the sides of the cavity, I tin them, and set
the diamond in position with a drop of soft solder.

Fig. 47.

In purchasing diamond bort, a good plan is to buy fragments that have
been employed in diamond drilling, and have become too small to reset;
in this case some idea as to the hardness of the bits may be obtained.
Full details as to diamond tool-making are given in books on
watch-making, and in Holtzapffell's great work on Mechanical
Manipulation; but the above notes are all that are really
necessary--it is, in fact, a very simple matter. The only advantage of
using a diamond tool for glass turning is that one does not need to be
always taking it out of the rest to sharpen it, which generally happens
with hard steel, especially if the work is turned a little too fast.

I recommend, therefore, that the student should boldly go to work
"free hand" with a hard file; but if he prefer the more formal
method, or distrust his skill (which he should not do), then let him
use a diamond point, even if he has the trouble of making it. When
using a diamond it is not necessary to employ a lubricant, but there
is some advantage in doing so.

The surface of the lens can be roughly shaped by turning to a template
or pattern made by cutting a circular arc (of the same radius as the
required surface) out of a bit of sheet zinc. Another very handy way
of making templates of great accuracy is to use a beam compass
(constructed from a light wooden bar) with a glazier's diamond instead
of a pencil. A bit of thin sheet glass is cut across with this
compass to the proper curvature--which can be done with considerable
accuracy and the two halves of the plate, after breaking along the
cut, are ground together with a view to avoiding slight local
irregularities, by means of a little fine emery and water laid between
the edges. In this process the glass is conveniently supported on a
clean board or slate, and the bits are rubbed backwards and forwards
against eac