_Method for Converting Negatives Directly into Positives_.--Captain
Bing, who is employed in the topographic studios of the Ministry of
War, has devised a process for the direct conversion of negatives into
positives. The idea is not a new one; but several experimenters, and
notably the late Thomas Sutton, have pointed out the means of effecting
this conversion; it has never, however, so far as I know, been
introduced into actual practice, as is now the case. The process which
I am about to describe is now worked in the studios of the Topographic
Service. The negative image is developed in the ordinary way, but the
development is carried much further than if it were to be used as an
ordinary negative. After developing and thoroughly washing, the negative
is placed on a black cloth with the collodion side downward, and exposed
to diffuse light for a time, which varies from a few seconds to two or
three minutes, according to the intensity of the plate. 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