The first experiments were carried out with : 200 solutions, in which the sensitive plate was bathed for I minute. At the end of this time the dark-room may be illuminated as brightly as required, provided the blue and violet radiations are absorbed by a yellow or orange filter. A yellow light-source of feeble intensity, such as a candle or petrol flame, may be brought quite close to the dish in which the development is taking place, and it is even possible to examine the negative by transmitted light as frequently and for as long as required during the course of development.
Almost immediately it was shown that desensitizing could be brought about with even the much more dilute solutions of x : 20,000 and i: 50,00o, so long as moderate intensity of illumination is observed in the dark-room, either as a bath before development or in the developer With either of the two methods of desensi tizing, safranine prevents or reduces considerably the development fog, 2 so that it is possible, when necessary, to extend development beyond the limits usually fixed by the growth of the fog, and also to use a much more alkaline developer. Finally, hydroquinone developer, which works somewhat slowly in the normal state, acquires almost the properties of a rapid developer, such as nietol.
The only defect of safranine is its strong staining action, which causes a considerable colouration of the fingers (more particularly the nails) and of the gelatine, especially if it has been used in the concentrations which were first recommended. The colouration of the gelatine is removed for the most part during the subsequent manipulations and the washing, any defects in this last process being very clearly shown by the unequal colouration of the different parts of the plate. This colouration, however, even when it is fairly intense or irregular, does not affect printing adversely, and may therefore be ignored.
The most useful employment of desensitizing is emphatically with panchromatic plates, which. in the past have had to be developed in prac tically complete darkness, but which now, after desensitizing in a preliminary bath, or in the developer itself, may be developed in a much brighter light than that previously used for ordinary emulsions.' All desensitizers are not, however, suited to all emulsions which are panchromatic, or sensi tive to infra-red, probably owing to reactions between the colour sensitizers and the desensi tizer. The effect of a given dye on a given emulsion varies greatly from one spectral region to another.
Many contradictions may be noted in publica tions on the properties of desentizers owing to the use in experiments of impure or wrongly labelled products. A large number of substances
which are energetic desensitizers in plain aqueous solutions are unusable in practice, the desensi tization being annulled or decreased during development by the action of one or other of the components of the tests of a desensitizer must therefore include a practical development test in an abundance of yellow light (Miss F. M. Hamer, 1931).
332. The systematic study of a large number of dyes undertaken by various workers has shown that the number of basic dyes which exercise this narcosis or numbing effect on silver bromide emulsions is very considerable. Although certain of them desensitize the emul sion, others cause fog. Others either desensitize or fog the emulsion, according to the concen tration of the solution employed. Methylene blue, for example, at a concentration of : r,000,000,000 (r grm. in L000 cubic metres of water), exerts an appreciable desensitizing action, although admittedly far from complete (at such a degree of dilution the solution is absolutely colourless), but at a concentration about a thousand times greater produces a marked amount of fog. Others weaken the latent image to a greater or less extent.
Among the desensitizers which do not belong to the safranine group we may mention the following: rhoduline red, auramine, chrysoidine, fuchsine (Liippo-Cramer), tolulyl red, (A. and L. Lumiere and A. Seyewetz, 1921), a green non-tinctorial mixture of secret com position, supplied commercially under the name of pinacryptol green (E. Koenig, 1922), which possesses desensitizing properties equal to those of the safranines, but which, at least with certain emulsions, increases the chemical fog instead of diminishing it ; finally, there is a red (lye, which has all the properties of safranine and, above all, stains the fingers, nails, and gelatine only very feebly, namely, basic scarlet N (Cie. Nat. des Matieres Colorantes), first recommended in 1924 by the Pathe-Cinema Laboratories. 3 The desensitizing power (ratio of the tivities before and after treatment) varies with the concentration. In concentrations of about 5,000 it is approximately ioo for safranine, pinacryptol green, and basic scarlet and for the other dyes mentioned' it is considerably lower. In very dilute solutions, the desensi- tizing power is markedly reduced; it is only Io for safranine in concentrations of about : 1,000,000.