o hard that they scratch the metal all over the surface
with fine microscopic scratches. We always work for figure, and when
we get a hard polisher that is in proper shape, we can do ever so many
surfaces with it if the environments of temperature are all right. If
we have fifty speculum flats to make, and we recently made three times
that number, we get them all ready and of accurate surface with the
hard polisher. Then we prepare a very soft polisher, easily indented
when cold with the thumb nail. A drop of rouge and about three drops
of water are put on the plate, and with the soft polisher about one
minute suffices to clean up all the scratches and leave a beautiful
black polish on the metal. This final touch is given by hand; if we
do not get the polish in a few minutes the surface is generally ruined
for shape, and we have to resort to the hard polisher again.

I assure you that nothing but patience and perseverance will master
the difficulty that one has to encounter, but with these two elements
'you are bound to get there.'"

CHAPTER III

MISCELLANEOUS PROCESSES

Sec. 74. Coating Glass with Aluminium and Soldering Aluminium.

A process of coating glass with aluminium has been lately discovered,
which, if I mistake not, may be of immense service in special cases
where a strongly adherent deposit is required. My attention was first
attracted to the matter by an article in the Archives des Sciences
physiques et naturelles de Geneve, 1894, by M. Margot. It appears
that clean aluminium used as a pencil will leave a mark on clean damp
glass. If, instead of a pencil, a small wheel of aluminium--say as
big as a halfpenny and three times as thick--is rotated on the lathe,
and a piece of glass pressed against it, the aluminium will form an
adherent, though not very continuous coating on the glass.

Working with a disc of the size described rotating about as fast as
for brass-turning, I covered about two square inches of glass surface
in about five minutes. The deposit was of very uneven thickness, but
was nearly all thick enough to be sensibly opaque. By burnishing the
brilliance is improved (I used an agate burnisher and oil), but a
little of the aluminium is rubbed off. The fact that the burnisher
does not entirely remove it is a sign of the strength of the adherence
which exists between the aluminium and the glass. In making the
experiment, care must be taken to have the glass quite clean--or at
all