the grinding must be regular.
The edge may be inspected under a microscope, and it must be perfectly
smooth and even before it will cut sections. A finishing touch may be
given on a leather strap, but it must be done skilfully, otherwise it
is better omitted.

The necessity for providing exceptionally keen and sharp edges arose
in the manufacture of phonographs, where the knife used to turn up the
wax cylinders must leave a perfectly smooth surface. In 1889 this was
being accomplished on an ivory lap fed with a trace of very fine
diamond dust.

I have had this method in mind as a possible solution of the
difficulty of razor-grinding, but have not tried it. I imagine one
would use a soft steel or ivory slip rubbed over with fine diamond
dust and oil by means of an agate. The lap used in the phonograph
works was rotated at a high speed.

Sec. 80. On the Production of Quartz Threads.

[Footnote: Since this was written an article on the same subject by
Mr. Boys appeared in the Electrician for 1896. The instructions
therein given are in accordance with what I had written, and I have
made no alteration in the text.]

In 1887 the important properties of fused quartz were discovered by
Mr. Vernon Boys (Philosophical Magazine, June 1887, p. 489, "On the
Production, Properties, and Some Suggested Uses of the Finest
Threads"). A detailed study of the properties of quartz threads was
made by Mr. Boys and communicated to the Society of Arts in 1889
(Journal of the Society of Arts, 1889). An independent study of the
subject was made by the present writer in 1889 (Philosophical
Magazine, July 1890, "On the Elastic Constants of Quartz Threads ").

There is also a paper in the Philosophical Magazine for 1894 (vol.
xxxvii. p. 463), by Mr. Boys, on "The Attachment of Quartz Fibres."
This paper also appeared in the Journal of the Physical Society at
about the same date, together with an interesting discussion of the
matter. In the American Journal, Electric Power, for 1894, there is a
series of articles by Professor Nichols on "Galvanometers," in which a
particular method of producing quartz threads is recommended. The
method was originally discovered by Mr. Boys, but he seems to have
made no use of it. A hunt through French and German literature on the
subject has disclosed nothing of interest--nothing indeed which
cannot be found in the papers mentioned.

Sec. 81. Quartz fibres have two great advantages over other