of clean archangel pitch, allowed to
cool slowly and even to rest for a day before the work is proceeded
with. The whole surface is then ground and polished as before.

The mirrors are now reversed, when they ought to nearly fit the tool
(assuming that flats are being made, and the fellow tool in all other
cases), and are recemented by pitch to the appropriate backing ground,
and polished. If very excellent results are required, these processes
may be preceded by a preliminary rough grinding of one surface, so
that the little discs will "sit" exactly on the tool surface, and not
run the risk of being strained by capillary forces in the pitch. We
have always found this necessary for really good results.

On removing such mirrors from the backing, they generally, more or
less lose their figure, becoming (in general fairly uniformly) more
concave or convex. About 5 per cent of the mirrors thus prepared will
be found almost perfect if the work has been well done, and the rest
will probably be very fair, unless the diameter is very large as
compared with the thickness. The best way of grinding and polishing
such large surfaces (nests 10 inches in diameter) is on a grinding
machine, such as will be described below. The polishing is best done
by means of paper, as before described.

Having occasion to require hitherto unapproached lightness and optical
accuracy in such mirrors, I got my assistant to try making them of
fused quartz, slices being cut by a diamond wheel from a rod of that
material. Chips of natural quartz were also obtained from broken
"pebble" spectacles, and these were worked at the same time. The
resulting mirrors were certainly superior to the best we could make
from glass, but the labour of grinding was greater, and the labour of
polishing less, than in the latter case. The pebble fragments gave
practically as good mirrors as the fused slices. For the future it
will be better always to make galvanometer mirrors from quartz
crystals. These may be easily sliced, as will be described in Sec. 74.
The slices are dressed on a grindstone according to instructions
already given for small lenses.

The silvering of these mirrors is a point of great importance. After
trying nearly every formula published, we have settled down to the
following.

A solution of pure crystallised nitrate of silver in distilled water
is made up to a strength of 125 grams of the salt per litre. This
forms the stock solutio