where the
back centre generally is, and observe the images of the flame by
reflection from both the lens surfaces. This method is very handy
with small lenses; the mandrel is turned, and the lens adjusted by
hand till the images are immovable. In both cases, of course, the
edge of the lens is turned or ground till it is truly circular, the
position of the lens remaining undisturbed on the chuck. If the edge
gauge has been properly used in the earlier stages of figuring, it
will be found that very little turning or grinding is requisite to
produce a true centering.

The particular defect due to want of centering in a lens may be
observed by using it as the objective of a telescope, and observing a
star slightly out of focus. The interference fringes will not be
concentric circles unless the lens is properly centred. I ought to
say that I have not looked into the theory of this, but have merely
taken it as a generally admitted fact. The diseases of lenses and the
modes of treating them are dealt with in a book by Messrs. Cook of
York, entitled On the Adjustment and Testing of Telescopic Objectives.

The final process of figuring will be dealt with later on (Sec. Sec. 66 and
67), as it applies not only to lenses but to mirrors, prisms, etc. If
the instructions given have been carefully carried out on a 2-inch
lens, it should perform fairly well, and possibly perfectly, without
any further adjustment of the glass.

Sec. 64. Preparation of Small Lenses, where great Accuracy is not of the
first Importance.

Such lenses may generally be made out of bits of good plate or sheet
glass, and are of constant use in the physical laboratory. They may
be purchased so cheaply, however, that only those who have the
misfortune to work in out-of-the-way places need be driven to make
them.

Suitable glass having been obtained and the curves calculated from the
index of refraction, as obtained by any of the ordinary methods
applicable to plates (the microscope method, in general, is quite good
enough), squares circumscribing the desired circles are cut out by the
help of a diamond. [Footnote: Glazebrook and Shaw's Practical
Physics, p. 383 (4th ed.).] The squares are roughly snipped by means
of a pair of pliers or spectacle-maker's shanks. The rough circles
are then mounted on the end of a brass or iron rod of rather greater
diameter than the finished lenses are to possess. This mounting is
best done by centering cem