lenses or mirrors the final
touches have to be given after the optical behaviour of the lens or
mirror has been tested with the telescope itself, and this process is
called "figuring." A book might easily be written on the optical
indications of various imperfections in a mirror or lens. Suffice it
to say here that a sufficiently skilled person will be able to decide
from an observation of the behaviour of a telescope whether a lens
will be improved by altering the curvature of one or all of the
surfaces.

A very small alteration will make a large difference in the optical
properties, so that in general "figuring" is done merely by using the
rouge polishing tool as an abrading tool, and causing it to alter the
curves in the manner already suggested for grinding. There are other
methods based on knocking squares out of the pitch-polisher so that
some parts of the glass may be more abraded than others.

The "figuring" and polishing may be done by hand just like the
grinding. There are machines, however, which can be made to execute
the proper motions, and a polisher is set in such a machine, and the
mechanical work done is by no means inconsiderable. In fact for
surfaces above six inches in diameter few people are strong enough to
work a polisher by hand owing to the intense adhesion between it and
the exactly fitting glass surface.

Such is a general outline of the processes required to produce a lens
or mirror. These processes will now be dealt with in much greater
detail, and a certain amount of repetition of the above will
unfortunately be necessary: the reader is asked to pardon this. It
will also be advisable for the reader to begin by reading the whole
account before he commences any particular operation. The reason for
this is that it has been desirable to keep to the main account as far
as possible without inserting special instructions for subsidiary
operations, however important they may be; consequently it may not
always be quite clear how the steps described are to be performed. It
will be found, however, that all necessary information is really
given, though perhaps not always exactly in the place the reader might
at first expect.

Sec. 54. All the discs that I have seen, come from the makers already
roughly ground on the edges to a circular figure--but occasionally the
figure is very rough indeed--and in some cases, especially if small
lenses have to be made, it is convenient to begin by cut