The quantity of India ink added to the albumen should be such as the
paper be black when coated, but, however, sufficiently transparent
for one to see the shadow of objects placed on the back of it, and
the coating should not be thick. This is important in order to
allow the light acting through the whole thickness of the
preparation when the paper is insolated under the cliche, for, if
the film be too opaque or too thick (by addition of too much gum
arabic), it would be only impressed on its surface, and the image
dissolved during the development. The cause of this failure must be
explained. Under the action of light the bichromate employed to
sensitize the albumen is reduced into chromic oxide which render
insoluble this organic substance--or any other, such as caseine,
gelatine, gum arabic, etc.; therefore whenever the film is not acted
on in its whole thickness, the subjacent part being still soluble,
is necessary washed off and with it the superficial impressed part,
that is, the image.

2d. Take 10 parts of lamp black and work it up in a mortar to the
consistency of a thin paste by gradually pouring a little of a
solution of from 6 to 8 parts of gum arabic and 1 part of liquid
glucose in 100 parts of water, adding afterwards the remainder, into
which 21/2 parts of ammonium bichromate have been dissolved, and
filter through flannel. With this, coat the paper by brushing so as
to form a thin and uniform film, and pin it up to dry in the dark.

These solutions keep well for a certain period. We have kept the albumen,
which we prefer to use, for two months in good condition; but the
sensitive paper does not for more than three or four days in taking the
usual care. It is more practical--and this is recommended--to leave out the
bichromate from the preparations, and to coat the paper, in quantity,
beforehand, and for use to sensitize it with a solution of potassium
bichromate at 31/2 per cent. of water applied on the verso with a Buckle
brush.(25)

The bichromate solution should be allowed to imbue the paper for about one
minute, and having brushed it once more, the paper is pinned up to dry in
the dark room. It can also be sensitized from the back by floating, if
this manner is found more convenient.

When dry the paper is impressed under a negative cliche of good intensity
until t