with fifty threads to the
inch.

It may perhaps be mentioned here that it is a pity to throw away emery
which has been used between glass and glass. In the chapter dealing
with fine optical work the use of emery of various grades of fineness
will be treated, and the finer grades can only be obtained (to my
knowledge) from emery which has been crushed in the process of glass
or metal grinding, especially the former. A large jam-pot covered
with a cardboard lid does well as a receptacle of washings.

Sec. 47. Glass-cutting.

This is an art about which more can be learned in five minutes by
watching it well practised than by pages of written description. My
advice to any one about to commence the practice of the art would be
to make friends with a glazier and see it done. What follows is
therefore on the supposition that this advice has been followed.

After some experience of cutters made of especially hardened steel, I
believe better work can generally be got out of a diamond, provided
the cost is not an objection. It is economy to pay a good price for a
good diamond. As is well known, the natural angle of the crystal
makes the best point, and a person buying a diamond should examine the
stone by the help of a lens, so as to see that this condition is
fulfilled. The natural angle is generally, if not always, bounded by
curved edges, which have a totally different appearance from the sharp
edges of a "splinter."

When a purchase is to be made, it is as well for the student to take a
bit of glass and a foot-rule with him, and to test the diamond before
it is taken away. When a good diamond has been procured, begin by
taking cuts on bits of clean window glass until the proper angle at
which to hold the tool is ascertained. Never try to cut over a
scratch, if you value your diamond, and never press hard on the glass;
a good cut is accompanied by an unmistakable ringing sound quite
different from the sound made when the diamond is only scratching.

Perhaps the most important advice that can be given is, Never lend the
diamond to anybody--under any circumstances.

The free use of a diamond is an art which the physicist will do well
to acquire, for quite a variety of apparatus may be made out of glass
strips, and the accuracy with which the glass breaks along a good cut
reduces such an operation as glass-box-making to a question of
accurate drawing.

Sec. 48. Cementing.

One of the matters which is gener