quares. For a small lens the iron or
lead backing may be used, for a large one the slate. The rough tools
are used to give an approximate figure both to the lens and to the
finishing tools.

The final adjustment is attained by grinding one of the glass-faced
tools alternately upon the lens and upon the fellow glass-faced tool.
The spherometer is accepted at all stages of the process as the final
arbiter as to curvature. Some hints on the form of strokes used in
grinding will be given later on (see Sec. 61). It suffices to state here
that the object throughout is to secure uniformity by allowing both
the work and the tool to rotate, and exercising no pressure by the
fingers. The tool backing may weigh from one to two pounds for a
2-inch lens.

Sec. 60. The tools and lens being all of the same curvature, the state
of the surface is gradually improved by grinding with finer and finer
emery. The best way of grading the emery is by washing it with clean
water, and allowing the emery (at first stirred up with the water) to
settle out. The longer the time required for this part of the process
the finer will be the emery deposited. An ordinary bedroom jug is a
very good utensil to employ during this process; a large glass jug is
even better. The following grades will be found sufficient, though I
daresay every operative's practice differs a little on this point.

1st grade: Flour emery, with the grit washed out, i.e. allowed to
stand for 2" (sec.) before being poured off.

2nd grade: Stand 5" (secs.), settle in 1' (min.)

3rd grade: Stand 1', settle in 10'.

4th grade: Stand 10', settle in 60'.

It is generally advisable to repeat the washing process with each
grade. Thus, selecting grade 2 for illustration, the liquor for grade
3 must be poured off without allowing any of the sediment to pass over
with it. If any sediment at all passes, one has no security against
its containing perhaps the largest particle in the jug. As soon as
the liquor for No. 3 has been decanted, jug No. 2 is filled up again
with clean water (filtered if necessary), and after standing 5" is
decanted into jug No. 2b, the sediment is returned to jug No. 1, and
the liquor, after standing 1', is transferred to jug No. 3.

The greatest care is necessary at each step of the operation to
prevent "sediment" passing over with liquor. There is a little danger
from the tendency which even comparatively large particles of emery
have to f