hang about one-quarter of an inch. The object of the
straight strokes is to counteract the tendency to a gradual
accumulation of the emery in the centre, which results from the
circular grinding.

A great deal of the art of the process consists in knowing how to work
the tool to produce any given effect. For instance, if the lens
requires to be ground down near the centre, the epicycloidal strokes
must be nearly central; the tool must never overhang very much. If,
on the other hand, it is the edges which require attention, these must
be dealt with by wider overhanging strokes. The tool must be
frequently tested on its fellow, and, indeed, ground upon it if any
marked unevenness of action (such as that just described) is required
for the lens. A check by spherometer will be applied at intervals
according to the judgment of the operator, but, in any case, the
fellow tool and lens should be kept at very nearly the same figure.

The emery should never be allowed to become anything like dry between
the tool and the lens, for in some way (probably by capillary action
increasing the pressure of the tool) this seems to lead to scratching
and "rolling" of the emery. The channels in the glass tool between
the squares are of the greatest importance in enabling the emery to
distribute itself. Perhaps the best guide in enabling one to judge
as to when it is time to wash off the emery and apply fresh is the
"feel" of the tool; also when the mud gets light in colour we know
that it is full of glass dust, and proportionately inoperative.

New emery may be put on, say, every five minutes, but no absolute rule
can be given, for much depends on the pressure of the tool upon the
lens. In the case considered a brass or lead, or even slate tool, of
an inch, or even less, in thickness, will press quite heavily enough.
In washing the lens and tool before new emery is introduced, a large
enamelled iron bucket is very handy; the whole of the tool should be
immersed and scrubbed with a nail-brush. The lens surface may be
wiped with a bit of clean sponge, free from grit, or even a clean damp
cloth.

When the time comes to alter the grade of emery, a fresh lot of
newspapers should be put down, and tools, lens, and pedestal well
washed and brushed by the nail-brush. The surfaces should be wiped
dry by a fresh piece of rag, and examined for scratches and also for
uniformity of appearance; a good opinion can be formed as to the fit
of