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408 PRINCIPLES OF CINEMATOGRAPHY
concentrated acetic acid instead of nitric acid, a material known as cellulose acetate is obtained. This material is relatively difficult to ignite and, if wound in roll form, will quickly become extinguished or merely smoulder rather like burning rags.
Reversal Stocks
The remaining requirements which must be met by a film suitable for amateurs were all satisfied by the introduction of the so-called 'reversal' emulsions. A reversal emulsion is one which, after the normal exposure in the camera, can be so treated during the processing operations that a positive image is finally obtained instead of the normal negative this positive image being on the identical film support which was used in the camera.
In the normal black-and-white reversal process the film is first developed to a negative in the usual manner but is then passed through a bleaching solution so designed that it is only capable of dissolving that silver which was previously developed the bleaching solution does not attack the undeveloped silver halide which, since it did not receive any exposure in the camera, could not be reduced to metallic silver by the developer. On leaving the bleaching solution the film will only carry unexposed silver bromide grains plus some of the solution itself and must, therefore, be thoroughly washed until all traces of the bleach are removed.
The silver bromide which remains is then exposed to white light and the film is passed through a second developing solution which, of course, produces an image complimentary to that originally obtained in the first developer, that is, it produces a positive image. The stages of reversal processing are more clearly seen from Figure 169 which shows cross-sectional views of an emulsion as it would appear at each stage of the process. Diagram 'A' represents an original subject which has been photographed and which consists of alternate rectangular areas of black and white material. Diagram 'B' indicates the condition of the film used to photograph this subject after it has passed through the first developing solution. Those small areas in the emulsion which become opaque correspond with similar small areas of white material in the original subject, whilst those areas of the subject which are black have had no effect upon corresponding areas of the emulsion which, therefore, remain as unaltered silver bromide grains.
The film is then passed through a bleaching solution which will dissolve all the metallic silver corresponding to those sections of the emulsion which received exposure and became developed. The condition of the film as it passes from the bleaching bath is seen