curved surfaces.

A good deal will depend on the method employed for supporting the
work; it is in general better to support the tool, which may have a
slate backing of any desired thickness, whereby the difficulty
resulting from strains is reduced. The work must be mounted in such a
way as to minimise the effect of changes of temperature. If a pitch
bed is selected, Mr. Brashear's instructions for rock salt may be
followed, with, of course, the obvious necessary modifications. See
also next section.

Sec. 73. Polishing Flat Surfaces on Glass or on Speculum Metal.

The above process may be employed for speculum metal, or pitch may be
used. In the latter case a fresh tool must be prepared every hour or
so, because the metal begins to strip and leave bits on the polisher;
this causes a certain amount of scratching to take place. As against
this disadvantage, the process of polishing, in so far as the state of
the surface is concerned, need not take an hour if the fine grinding
has been well done.

For the finest work changes of temperature, as in the case of glass,
cause a good deal of trouble, and the operator must try to arrange his
method of holding the object so as to give rise to the least possible
communication of heat from the hand.

The partial elasticity of paper, which is its defect as a polishing
backing, is, I believe, partly counterbalanced by the difficulty of
forming with pitch an exact counterpart tool without introducing a
serious rise of temperature (i.e. warming the pitch). The rate of
subsidence of the latter is very slow at temperatures where it is hard
enough to work reliably as a polisher.

A student interested in the matter of flat surfaces will do well to
read an account of Lord Rayleigh's work on the subject, Nature, vol.
xlviii, 1893, pp. 212, 526 (or B. A. Reports, 1893). In the first
of these communications Lord Rayleigh describes the method of using
test plates, and shows how to obtain the interference fringes in the
clearest manner.

For the ordinary optician a dark room and a soda flame afford all
requisite information; and if a person succeeds in making three glass
discs, say 6 inches in diameter, so flat that, when superposed in any
manner, the interference fringes are parallel and equidistant, even to
the roughest observation, he has nothing to learn from any book ever
written on glass polishing. Lord Rayleigh has also shown how to use
the free clean surface of wa