For an investigation on sensitivity centers, densities developed aficer fixation were desired. This method has the disadvantage of giving much smaller densities at the same exposures than the usual development, since, on the one hand, the latent image is partly dissolved in the fixing bath of thiosulfate, and on the other hand, sulfide fog is produced. Bullock, 1 therefore, used sodium sulfite for the fixing bath instead of sodium thiosulfate. Even so, the threshold speed was only 0.2 of the threshold speed produced by complete chemical development. Besides, the fixing process with sodium sulfite requires many hours or even several days. It seemed advisable, therefore, to return to thiosulfate as a fixing bath, using such modifications as had been suggested by previous investigators.
The dissolving power of thiosulfate for the latent image depends on the pH of the solution; this has been pointed out already by Lumi~re and Seyewetz, 2 and Eggert and Noddack 3 showed that even the developed image is dissolved in thiosulfate, but that this reaction can be greatly decreased by the addition of alkali. Meidinger ~ was able to increase the density of the developed image considerably by using for the preceding fixation a solution of thiosulfate which was either of very low concentration or was strongly alkaline. Recently, Kempf 5 found an optimum density with a solution containing o.5 per cent. thiosulfate and o.5 per cent. sodium hydroxide.