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Desensitizing of Photographic Emulsions 329

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DESENSITIZING OF PHOTOGRAPHIC EMULSIONS 329. Use of Coloured Developers. Carey-Lea, as far back as 1877, showed that after immersing a photographic plate in ferrous oxalate developer it was possible, without risk of fog, to illuminate the dark-room much more brightly than would have been possible during the handling of the same plate when dry. This effect was attributed to the orange-red colouration of the developer," and little attention was paid to it, since the emulsions were so insensitive that in all cases a bright illumination could be used.

About 1889 various attempts were made to introduce into photographic practice the use of developers which had been coloured red by the addition of certain dyes, these dyes being subsequently destroyed in an acid fixing bath (coralline, croceine). The method, however, was not successful, since the dyes transmitted both blue and violet light. In 1900 A. and L. Lumiere and A. Seyewetz, with the same object in view, recommended the use of an orange coloured compound, magnesium picrate, which allows the development to be observed at a distance of about x8 in. from a candle or at about 5 ft. from a If) c.p. lamp, provided development is sufficiently rapid and the plate remains covered by a layer of developer about in. thick. If these distances are doubled, it is possible to remove the negative from the bath and examine it very quickly by transmitted light. This method of working, although it afforded interesting demonstrations, was not used in practice.

330. Loss in Sensitivity of Emulsions Impreg nated with Developer. After the use of organic developing agents had become general, it was frequently pointed out that after the developing bath had thoroughly impregnated the emulsion a considerable lowering of sensitivity took place, which could not be explained in these cases by any colouration of the bath. In 1901 Liippo Cramer carried out some experiments in this direction (using the different developing agents) and discovered that this action occurred with nearly all the most common developers (with the exception of hydroquinone), both in plain and alkaline solutions, and that sulphite tended to reduce the effect. In 1920 the same worker

found a very marked reduction in the sensitivity of photographic emulsions' (reduced to 1/50th or Ifiooth of the original value) after they had been bathed for about I minute in a pure solution of diaminophenol-hydrochloride of from 0-02 to o-o5 per cent strength, although no reduction in the latent image resulted from this treatment. An analogous effect may be obtained by the addition of this product to a hydro quinone developer ; it was soon recognized that this desensitizing was due to traces of oxidation products, which are rapidly formed by the action of the air on dilute solutions of this developer.

331. Desensitizers. During further experi ments on this phenomenon, Luppo Cramer dis covered, several months later, the remarkable desensitizing properties of phenosafranine, a violet-red dye of considerable tinctorial power.

It was soon shown that the desensitizing pro perties of phenosafranine are possessed by various safranines and, in varying degree, by different substances of similar constitution. The red colour of some of these substances might lead to the supposition that the protection from fog is due to the absorption of the active radiations by the dye impregnating the emul sion. This is not the case, however, for the efficiency of these desensitizers is practically as great with panchromatic as with ordinary emulsions, and, secondly, a sensitive plate exposed behind a cell containing a solution of the dye, even in concentration greater than that used for desensitization, and under a laver of greater thickness than that of the developing bath, develops an intense fog. This is clue to the fact that the red safranines absorb very little of the violet radiations. It was discovered not long afterwards that certain violet dyes belonging to the safranine family were capable of acting as efficient desensitizers.

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