others out of
softer pitch. The aim is to produce a polishing tool which will
polish unequally so as to remove the glass chiefly from predetermined
parts of the lens surface. The tool is worked over the surface of the
lens by the polishing machine, and part of the art consists in
adjusting the strokes to assist in the production of the local
variations required.

A source of difficulty and danger lies in the fact that the pitch
squares are rarely of the same hardness, so that some abrade the glass
more rapidly than others. This is particularly likely to occur if the
pitch has been overheated. [Footnote: When pitch is heated till it
evolves bubbles of gas its hardness increases with the duration of the
process.] The reader must be good enough to regard these remarks as
of the barest possible kind, and not intended to convey more than a
general idea of the nature of the process of figuring.

Sec. 68. A few remarks on cleaning lenses will fittingly close this part
of the subject. There is no need to go beyond the following
instructions given by Mr. Brashear in Popular Astronomy, 1894, which
are reproduced here verbatim.

"The writer does not advise the use of either fine chamois skin,
tissue paper, or an old soft silk handkerchief, nor any other such
material to wipe the lenses, as is usually advised. It is not,
however, these wiping materials that do the mischief, but the dust
particles on the lenses, many of them perhaps of a silicious nature,
which are always harder than optical glass, and as these particles
attach themselves to the wiping material they cut microscopic or
greater scratches on the surfaces of the objective in the process of
wiping.

"I write this article with the hope of helping to solve this
apparently difficult problem, but which in reality is a very simple
one.

"Let us commence by taking the object glass out of its cell. Take out
the screws that hold the ring in place, and lift out the ring.
Placing the fingers of both hands so as to grasp the objective on
opposite sides, reverse the cell, and with the thumbs gently press the
objective squarely out of the cell on to a book, block of wood, or
anything a little less in diameter than the objective, which has had a
cushion of muslin or any soft substance laid upon it. One person can
thus handle any objective up to 12 inches in diameter.

"Before separating the lenses it should be carefully noted how they
were put together with relation