old file, and the polisher reapplied to the glass. A few
strokes will develop other points, more numerous than before, and
these in turn must be erased. The process is continued till the whole
surface of the polishing tool is evenly covered with bright specks,
and then the polishing may be proceeded with. The specks should not
be more than about one-eighth of an inch apart, or the polishing will
be irregular.

The operation of polishing is similar to that of grinding. A
reasonable time for polishing a glass surface is twenty hours; if
more time is required it is a sign that the fine grinding has not been
carried far enough. The progress of the operation may be best watched
by looking at the surface--not through it. For this purpose a good
light is requisite. When the lens is dismounted it may be examined by
a beam of sunlight in a dark room, under which circumstances the
faintest signs of grayness are easily discernible.

It may be mentioned here that if the surface is in any way scratched
the rouge will lodge in the scratches with great persistence, and an
expert can generally tell from the appearance of scratches what kind
of polishing powder has been employed.

The persistence with which rouge clings to a rough surface of glass is
rather remarkable. Some glass polishers prefer to use putty powder as
a polishing material, and it is sometimes said to act more quickly
than rouge; from my rather limited experience I have not found this
to be the case, but it may have merits that I do not know of. Is it
possible that its recommendation lies in the fact that it does not
render scratches so obtrusively obvious as rouge does?

Rouge is generally made in two or more grades. The softer grade is
used for polishing silver, and is called jewellers' rouge. The harder
grade, suitable for glass polishing, is best obtained from practical
opticians (not mere sellers of optical instruments). I mean people
like Messrs. Cook of York. Many years ago I prepared my own hard
rouge by precipitating ferrous sulphate solution by aqueous ammonia,
washing the precipitate, and heating it to a red heat. The product
was ground up with water, and washed to get rid of large particles.
This answered every purpose, and I could not find that it was in any
way inferior to hard rouge as purchased. The same precipitate heated
to a lower temperature is said to furnish a softer variety of rouge;
at all events, it gives one more suitable