ors them olive green.

Transparencies for lantern slides, etc., are best colored with the
couleurs a l'albumine of L. Encausse, sold by J. Reygondaud, Paris
(France). They are transparent.(33)

_Toning and Intensifying.--_The carbon proofs can be toned and at the same
time intensified by reagents acting with chromic oxide.

The dyes or coloring matters precipitated are not opaque, and, as a
consequence, not objectionable for transparencies. The following
processes are the most employed:

Prepare three solutions as follows:

A. Ferric sulphate 5 parts
Water 100 parts
B. Sodium carbonate 2 parts
Water 100 parts
C. Gallic acid 5 parts
Water 100 parts

Dissolve the gallic acid in warm water. Filter each solution. They keep
well.

To tone, the plate is immersed for, say, ten minutes in A, then, after
rinsing slightly, it is placed in B for the same period, rinsed again and
flowed with C until the desired color is obtained. The tone is a splendid
purple black color. If a solution of pyrogallol be substituted to that of
gallic acid, the tone is green, and to a green bordering to black when a
solution of catechu is used, the catechu exerting at the same time a
tanning action on the gelatine. After toning, the plate should be
thoroughly washed.

A similar process consists to wet the plate under the tap, then to flow
over a mixture by about equal volumes of

A. Ferrous sulphate 5 parts
Acetic acid, No. 8 5 parts
Water 100 parts, filter
B. Gallic acid 5 parts
Water 100 parts

When toned, the plate is well washed, then flowed once with the alum
solution and again washed. The tone by this process easily turns to an
inky blue not very agreeable. The action should be stopped a little
before the desired color is obtained.

It sometimes happens that the image in drying intensifies more then
necessary. It can be cleared with a solution of oxalic or citric acid.

A brown sepia is obtained by toning first with potassium permanganate, 1
per cent. of water, then, after washing, with a solution of pyrogallol.
If gallic acid be used instead of pyrogallol, the tone is black. By this
process a great intensity is obtained. A dilute solution of ammonium
sulphide can be employed as a clearing agent.

Pyrogallol and silver nitrate give a warm black tone.

Potassium bichromate follow