pt to correct a,
defect, as will be explained) should the tool overhang the lens
surface by more than about one quarter the diameter of the latter.
After grinding say for an hour with one size of emery fed in by means
of a clean stick say every five minutes, the emery is washed off, and
everything carefully cleaned. The process is then repeated with finer
emery, and so on.

The different grades of emery are prepared by taking advantage of the
fact that the smaller the particles the longer do they remain
suspended in water. Some emery mud from a "roughing" operation is
stirred up with plenty of water and left a few seconds to settle, the
liquor is then decanted to a second jug and left say for double the
time, say ten seconds; it is decanted again, and so on till four or
five grades of emery have been accumulated, each jug containing finer
emery than its predecessor in the process.

It is not much use using emery which takes more than half an hour to
settle in an ordinary bedroom jug. What remains in the liquid to be
decanted is mostly glass mud and not emery at all. The process of
fine grinding is continually checked by the spherometer, and the art
consists in knowing how to move the grinding tool so as to make the
lens surface more or less curved. In general it may be said that if
the tool is moved in small sweeps, and not allowed to overhang much,
the Centre of the lens will be more abraded, while if bold free
strokes are taken with much overhanging, the edges of the lens will be
more ground away.

By the exercise of patience and perseverance any one will succeed in
gradually fine grinding the lens surface and keeping it to the
spherometer, but the skill comes in doing this rapidly by varying the
shape of the strokes before any appreciable alteration of curvature
has come about.

Polishing.

The most simple way of polishing is to coat the grinding tool with
paper, as will be described, and then to brush some rouge into the
paper. The polisher is moved over the work in much the same way as
the fine grinding tool, until the glass is polished. Many operators
prefer to use a tool made by squeezing a disc of slate, armed with
squares of warm pitch, against the lens surface (finely ground), and
then covering these squares with rouge and water instead of emery and
water as in the fine grinding process.

The final process is called "figuring." It will in general be
unnecessary with a small lens. With large