Instead of dissolving out the silver bromide whilst allowing the reduced metallic silver to remain, the silver may be dissolved out and the silver bromide left. When this silver bromide is reduced a direct positive image is obtained, i.e. an image in which the distribution of opacities and transparencies is the inverse of those in the normal negative. In this way is obtained a direct positive, which, however, is not usually very good, unless emulsions in very thin and perfectly uniform layers are used.
Physical development of gelatino-bromide emulsions may be carried out before or after fixing, but as the coating does not contain any soluble silver salts, a solution of silver nitrate needs to be added to the developer.
Lastly, the possibility should be mentioned of carrying out development and fixing simul taneously, by mixing in suitable proportions a suitable developer and sodium thiosulphate.
This procedure is, however, of no practical importance.
The processes mentioned above must ob viously be carried out in the absence of all light which can act on the sensitive materials used. It will be seen later that a very weak orange-red or green light is generally employed.
Another method is to desensitize the emulsion, after exposure in the camera, by treatment in a very dilute solution of certain substances, coloured or non-coloured, the actual coloration, however, playing no part. A desensitized emul sion may be developed in ample yellow light, or even in weak white light.
The negative obtained by the normal pro cesses does not always possess exactly the desired qualities. Its contrasts may not be sufficient, in which case it is necessary to intensify it. Its opacities may be too great and it may then be advantageous to submit it to the process of reduction,' which, according to the method adopted, may either increase or decrease the contrasts, without, however, always acting to the same degree on all the various tones.
Local defects in the negative or in the subject, or in the distribution of lights and shadows in the subject, may call for spoiling, retouching, or working-up. Finally, it may be necessary to separate the gelatine film containing the image from its support, especially from the glass on which it was coated, with the object of reversing the image as regards right and left, this process being known as stripping.