The chief disadvantage of this method is the total lack of latitude in exposure and, indeed, the impossibility of rendering correctly, what ever the exposure, a subject of which the extreme luminosities slightly exceed the range permitted. Owing to the fact that the content of silver bromide per unit of surface has been reduced in order to enhance rapidity, contrast would, in fact, be reduced if a more contrasty emulsion were not used.
After dissolving the first image, clearing with a bath of bisulphite (§ 439) and rinsing, the residual silver bromide is placed in a developer in a weak light, or after a fairly ample exposure to light, or finally after it has been acted upon by a solution causing a very strong It can also be treated with a solution reducing the silver halide, or by a solution of a sulphide or by a sulphurizing mixture (§ 589).
To permit the use of thicker emulsions and then correct under-exposure, R. E. Liesegang (1925) proposed giving to the film, before development, an auxiliary exposure such that the sum of the intermediate image itself and the general fog so produced uses up the whole of the silver bromide of the emulsion in those regions corresponding with the high-lights. The amount of the auxiliary exposure is determined by systematic trials on images similar to those which are to be reversed.
442. Reversal by Means of Residual Silver Bromide of which the Excess is Dissolved to an Accurately Determined Degree. In the method of reversal by total blackening of the residual silver bromide the only correct exposure is that which allows the first development to reduce all the silver bromide in the image of the whites of the and only in the image of these whites. A shorter exposure would leave in these areas available silver bromide and would, there fore, represent the whites by a greater or less density of the final image. Such an under exposure can be corrected after dissolving the silver of the first image by dissolving a certain amount of the residual silver bromide (J. G. Capstaff, 1923). The image is immersed for an accurately determined time in a dilute fixing solution, the operation being adjusted so that no silver bromide remains in the regions corre sponding with the high-lights of the final image the best conditions are determined by trials with images which are identical with those which are to be reversed. 3 A variant of this method, which is really of older date, for it was specified in the instruc tions for treatment of the Autochrome plate (L. Lumiere, 1906), consists in effecting the first development in a developer containing a solvent, usually ammonia, but sometimes also hyposulphite, sulphocyanate or cyanide. A strong developer is used in which development is complete in about 2 minutes, after which prolongation of immersion in the bath merely eliminates gradually the silver halide, which according to the composition of the developer remains in solution or is deposited both on the plate and on the sides of the dish as metallic silver. The total blackening is thus progres
sively decreased, thus approximating to the maximum developable density, and thereby permitting pure whites to be obtained in the reversed image. It is understood that in this particular case under-exposure may be com pensated by a prolongation of the time of development. The presence of the solvent is equivalent in some degree to an increase of the sensitivity of the emulsion, reckoned in terms of the final positive image (L. Lobel, J. Lefevre, M. Dubois, and J. Vidal, 1927-28).
It must be remembered that, on account of reversal, the characteristics of the resultant image are, in case of faulty working, very different from those ordinarily arising from the same errors. Thus, fog in the first image results in weakness or absence of the final image. Insufficient density corresponds with over exposure or over-development of the first image, whilst a very dark image, even in the " whites," corresponds with under-exposure or under development of the intermediate image. It is preferable to use baths of which the formulae are recommended by the maker of the emulsion employed.
443. Various Other Methods. By means of the Herschel effect (§ 197) it is possible to obtain reversed images on transparency plates. The plate is uniformly fogged in white light and then exposed in red light under the negative to be copied. The time of exposure to red light can be much shortened by impregnating the emulsion with a desensitizing dye and potassium bromide after the uniform exposure to white light.
A. and L. Lumiere and A. Seyewetz (x911) suggested the following procedure : after re versing, washing, and prolonged exposure to light, dissolve the silver bromide and develop by the method of development after fixation (§ 396) When it is required to produce, not direct posi tives from original subjects, but negative copies of negatives (direct reproductions) or direct posi tives by printing from a positive transparency, very various methods may be employed. Their application presents no difficulty except, per haps, from the industrial standpoint of the slowness of printing. Notable among these methods of printing are the dusting-on process, employing a coating of bichromated syrup (§ 678), as used also in making photographic enamels the hydrotype methods, which make use of the differences of permeability ot a layer of bichromated gelatine after exposure to light under a photographic image, so that certain colouring matters are fixed only by those parts of the layer which have been more or less protected against light-action, and in a way almost proportional to the density of the image used as original (§ 675) also diazotype (§ 693).