At the usual solution strengths desensitiza. tion increases with the duration of the treat ment, first very rapidly and then more and more slowly. Prolonged washing of a desensitized photographic emulsion diminishes the desensi tization without annulling it.
333. Practical Methods of Desensitizing. In order to obtain the maximum advantages from the use of desensitizing methods, there should be two systems of illumination in the dark room, with arrangements for changing rapidly from one to the other (different lamps or different current supply), or an arrangement by means of which an additional safe light may be removed from the front of a permanent yellow screen. It should be pointed out that, even in a dark room different parts of which are being used for different operations (many operators working separately, or the treatment of cinematograph films in continuous machines), the elimination of oxidation fog is a sufficient advantage in itself to make the adoption of desensitizing, either before or during development, worth while. 2 When choosing a desensitizer, it is necessary to remember that coloured desensitizers, even those which are most easily removed, are generally held very strongly by paper fibres. It is therefore necessary, for the treatment of paper negatives, to choose either a colourless desensitizer, or one which has already been shown to be readily eliminated. If the desen sitizer is miscible with the particular developer in use, less efficient desensitization is obtained, for equal concentrations, by the use of a com bined bath than by preliminary treatment before development (Miss F. M. Hamer, 1929),
although the reverse has frequently been stated.. In all those cases, however, in which the sensi tive layer is wetted previous to development (cinematograph films are treated in this manner, to prevent the adherence of air bubbles) the desensitizing may be combined with this pre liminary treatment.' The desensitizer may be employed in solution of increasing dilution according as the intensity of illumination in the dark-room is cut down.
Whatever method is used for the use of the desensitizer, one minute must be allowed for its penetration through the layer of an ordinary plate or film, and a little longer in the case of multiple-coated emulsions and very thick layers.
Basic Scarlet N i : 5,00o gives excellent results, the solution requiring to be frequently renewed. It is made up by diluting a stock : soo solution, which latter keeps well if a little formaline (about 20 min. per 20 oz.) be added.
It is necessary to make allowance for the fact that desensitizing usually modifies the time of development required to reach a given degree of contrast, and affects the time of appearance of the image and the time required for its complete development to a different extent (sometimes in an inverse manner). The method of estimating the time of development from a measurement of the time of appearance of the first details of the image (Watkins method, § 384), is only applicable if the desensitizer is added to the developer.'