to recess the facets, provided hard
pitch is used as the cement. See note on hard pitch.] To save
trouble, it is usual, to make such lenses of equal curvature on both
faces; but of course this is a matter of taste.

Fig. 56.

For very common work, bits of good plate glass are employed, and the
manufacturer's surface treated as flat (Fig. 56). In this way
plano-convex lenses are easily and cheaply made. Finally the lenses
have to be centred, an essential operation in this case. This is
easily done by the reflection method--the edge being turned off by
the file and kerosene and the centering cement being used in making
the preliminary adjustment on the chuck. I presume a lens made in
this way is worth about a shilling, so that laboratory manufacture is
not very remunerative. Fig. 56 shows the method of mounting small
lenses for lathe grinding, when only one lens is required. The tool
is generally rotated in the lathe and the lens held against it.

Sec. 65. Preparing Small Mirrors for Galvanometers.

To get good mirrors for galvanometers, I have found the best plan is
to grind and polish a large number together, on a disc perhaps 8 or 10
inches in diameter. I was led to this after inspecting and rejecting
four ounces of microscope cover slips, a most wearisome process. That
regular cover slips should be few and far between is not unlikely,
seeing that they are made (by one eminent firm at least) simply by
"pot" blowing a huge thin bulb, and then smashing it on the floor and
selecting the fragments. As in the case of large mirrors, it is of
course only necessary to grind one side of the glass, theoretically at
all events. The objections to this course are:

(1) A silver surface cannot, in my experience, be polished externally
(on a minute object like a cover slip) to be anything like so bright
as the silver surface next the glass; and,

(2) if one side only is ground, it will be found that the little
mirror hopelessly loses its figure directly it is detached from the
support on which it has been worked. Consequently, I recommend that
these small mirrors should be ground and polished on both
sides--enough may be made at one operation to last for a very long time.

A slate back is prepared of the same radius of curvature as it is
desired to impart to the mirrors. Bits of thin sheet glass are then
ground circular as described in the last section and cemented to this
surface by the smallest quantity