strong commercial hydrochloric
acid to remove iron, wash with distilled water and a few drops of
alcohol, dry on blotting paper in the sun or otherwise. Put the dry
glass into a bottle or beaker, and begin by adding almond oil (or
bromo-napthalene), then add nut oil (or acetone) till the glass
practically disappears when examined by sodium light, or light of any
other wave-length, as may be required.

The adjustment of the mixture is a matter of great delicacy, one drop
too much of either constituent, in, say, 50 cubic centimetres, makes
all the difference. The final adjustment is best accomplished by
having two mixtures of the oils, one just too rich in almond, the
other in nut oil; by adding one or other of these, the required
mixture is soon obtained.

It is to be noted

(1) That adjustment is only perfect for light of one wave-length.

(2) That adjustment is only perfect at one temperature.

On examining a bottle of rather larger fragments of glass immersed in
an adjusted mixture by ordinary daylight, a peculiarly beautiful play
of colours is seen.

Of course, if it is only desired to make ground glass fairly
transparent, these precautions are unnecessary, but it seemed better
to dispose of the matter once for all in this connection.

M. Cornu's object was to make a varnish which would prevent reflection
from the back of a photographic plate on to the film. I have had
occasion to require to do the same when using a scale made by cutting
lines through a film of black varnish on a slip of glass. This
succeeded perfectly by making the varnish out of Canada balsam stained
with a black aniline dye.

Mr. Russell, Government Astronomer of New South Wales, finds that the
"halation" of star photographs can be prevented by pouring over the
back of the plate a film of collodion suitably stained.

Sec. 46. Making Ground Glass.

This is easily done by rubbing the surface of polished glass with a
bit of cast iron and washed "flour of emery." Of course, if the
fineness of grain of the surface is of importance, appropriate sizes
of emery must be employed. The iron may be replaced by a bit of glass
cut with transverse grooves to allow the emery to distribute itself,
or even by a bit of glass without such grooves, provided it does not
measure more than one or two inches each way. If great speed is an
object rather than the fineness of the surface, use a bit of lead and
coarse emery, say any that will pass a sieve