loat, in consequence of their refusing to get wet, and the
emery worked up on the side of the jug is also a source of danger,
therefore wipe the jug round inside before decanting.

In order to get a uniform grade stop the currents of water in the jug,
which may work up coarse particles, by holding a thin bit of wood in
the rotating liquid for a moment, and then gently withdrawing it in
its own plane. These precautions are particularly necessary in the
case of grades Nos. 2, 3, and 4, especially No. 4, for if a single
coarse particle gets on the tool when the work has progressed up to
this point it will probably necessitate a return to grinding by means
of No. 2, and involve many hours' work.

The surface of the lens will require to be ground continuously with
each grade till it has the uniform state of roughness corresponding to
the grade in question. Two hours for each grade is about the usual
time required in working such a lens as is here contemplated.

The coarser grades of emery may be obtained by washing ordinary flour
of emery, but the finer ones have to be got from emery which has been
used in the previous processes. It is not a good plan to wash the
finer grades of emery out of the proceeds of very rough grinding say
with anything coarser than flour of emery--as there is a danger of
thereby contaminating the finer grades with comparatively coarse glass
particles (owing to their lightness) and this may lead to scratching.
If the finer grades are very light in colour, it may be inferred that
a considerable portion of the dust is composed of glass, and this does
no good. Consequently time may be saved by stirring up the
light-coloured mass with a little hydrofluoric acid in a platinum
capsule; this dissolves the finely divided glass almost
instantaneously. The emery and excess of hydrofluoric acid may then
be thrown into a large beaker of clean water and washed several times.
Fine emery thus treated has much the same dark chocolate colour as the
coarser varieties.

The operator should not wear a coat, and should have his arms bare
while working with fine emery, for a workshop coat is sure to have
gathered a good deal of dust, and increases the chances of coarse
particles getting between the surfaces.

Sec. 61. Details of the Process of Fine Grinding.

A lens of the size selected for description is mounted as before
mentioned on a leaden pedestal, and the operator places the latter on
a table of co