nvenient height in a room as free from dust as possible.
Everything should be as clean as a pin, and no splashes of emery mud
should be allowed to lie about. I have found it convenient to spread
clean newspapers on the table and floor, and to wear clean linen
clothes, which do not pick up dust. I have an idea that in large
workshops some simpler means of avoiding scratches must have been
discovered, but I can only give the results of my own experience. I
never successfully avoided scratches till I adopted the precautions
mentioned.

Fig. 52.

The left hand should be employed in rotating the pedestal either
continuously (though slowly) or at intervals of, say, one minute.
This point is rather important. Some operators require two hands to
work the grinding tool, and in any case this is the safer practice.
Under these circumstances the pedestal may be rotated through
one-eighth or tenth of a revolution every three minutes, or
thereabouts. The general motion given to the grinding tool should be
a series of circular sweeps of about one-fourth the diameter of the
glass disc, and gradually carried round an imaginary circle drawn on
the surface of the lens and concentric with it (Fig. 52).

The tool may overhang the lens by a quarter of the diameter of the
latter as a maximum. The circuit may be completed in from twelve to
thirty sweeps. The grinding tool should be lightly held by the
fingers and the necessary force applied parallel to the surface. The
tool itself must be slowly rotated about its axis of figure. If the
tool be lightly held, it will be found that it tends to rotate by
itself. I say "tends to rotate," for if the tool be touching evenly
all over the surface it will rotate in a direction opposite to the
direction of the circular sweep. For instance, if the tool be carried
round its looped path clockwise, it will tend to rotate about its own
axis of figure counter-clockwise. If it touch more in the middle,
this rotation will be increased, while if it touches more along the
edge, the rotation will be diminished, or even reversed in an extreme
case.

Every fifty sweeps or so the tool should be simply ground backwards
and forwards along a diameter of the lens surface. This grinding
should consist of three or four journeys to and fro along, say, eight
different diameters. About one-quarter of the whole grinding should
be accomplished by short straight strokes, during which the tool
should only over