Afterward the
conversion is effected by moistening the plate afresh, and then plunging
it into a bath which is thus composed:
Water 700 cub. cents.
Potassium bichromate 30 grams.
Pure nitric acid 300 cub. cents.
In a few minutes this solution will dissolve all the reduced silver
forming the negative; the negative image is therefore entirely
destroyed; but it has served to impress on the sensitive film beneath
it a positive image, which is still in a latent condition. It must,
therefore, be developed, and to do this, the film is treated with a
solution of--
Water 1,000 grams
Pyrogallic acid 25 "
Citric acid 20 "
Alcohol of 36 deg. 50 cub. cents.
The process is carried on exactly as if developing an ordinary negative;
but the action of the developer is stopped at the precise moment when
the positive has acquired intensity sufficient for the purpose for which
it is to be used. Fixing, varnishing, etc., are then carried on the
usual way. The great advantage of this process consists in the fact of
its rendering positives of much greater delicacy than those that are
taken by contact; and, on the other hand, by means of it we are able to
avoid two distinct operations, when for certain kinds of work we require
positive plates where a negative would be of no service. M. V. Rau,
the assistant who has carried out this process under the direction of
Captain Bing, has described it in a work which has just been published
by M.
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