It is obvious that reversal cannot be applied to plates or films coated with several super posed emulsions ; the best results are generally obtained with sensitive material specially pre pared for this treatment.
Whatever the working method employed, it is essential that the first development be thorough, for the undeveloped latent negative image would be superimposed on the positive image, being able even to cause a reversal in the image of the high-lights.
439. Dissolution of the First Image. The re moval of the reduced silver is done preferably by means of an acid solution of permanganate (R. Namias, 1902), other silver solvents, and especially those containing chromic acid or bichromates in acid solution, constituting much more energetic The reversing bath is made up when required by mixing equal volumes of the following stable solutions— Potassium permanganate . 35 gr.
(4 gr..) Water, to make • . . 20 OZ.
(r,ocao c.c.) Sulphuric acid, 66° Be. 2 . 3f dr.
Water, to make . . 20 oz.
(r,oco c c.) The plate (or film) is placed in this bath, and the dish is rocked until the silver forming the first image has completely disappeared. The solution must not have a temperature above 68° F., otherwise it may attack the gelatine. When the whole of the silver seems to be dis solved, the plate is quickly rinsed and is then placed in a solution containing 2 per cent to 5 per cent of bisulphite of soda, in order to dissolve the brown manganese dioxide which has been formed in the gelatine, and to restore a normal sensitivity to the silver bromide desensitized by the treatment with permanga nate. If it is found that traces of the first image still remain it is necessary to repeat the operations.
Some workers, however, prefer to dissolve the image in a sulphuric solution of bichromate (1 per cent of potassium bichromate and i per cent of sulphuric acid, this latter being measured in volumes). The image thus reversed is, when finished, less contrasty than an image reversed with permanganate (C. Emmermann and K. Brandt, 1928). The bisulphite solution may then be replaced by a 5 per cent neutral sulphite solution.
For reversal in hot climates, the use has been suggested (A. Seyewetz, 1929) of the solution of eerie sulphate specified (§ 461) as a reducer, this solution having the advantage of hardening the gelatine.
440. Reversal by a Second, Determined Expo sure. This method of operation, which in fact is equivalent to making a positive copy (§ 437 can produce a positive with pure whites whatever the amount of silver bromide available in the areas corresponding to the pure whites, if the uniform given to the emulsion after dissolution of the silver and re-sensitiza tion, is suitably chosen.
The average speed of the residual emulsion, always less than its original speed, is increased since the first development, owing to a shorter first exposure, has affected a smaller number of sensitive grains. The necessary exposure depends both on the extreme luminosities of the subject and on the conditions of the first exposure. In the case of a subject of which the extreme luminosities have a range of 32 to 1, a suitable choice of the second exposure enables positives to be obtained which are practically identical with values of the first exposure within a range of 16 to I.
In the absence of an automatic regulator of the second exposure (J. G. Capstaff, 1927), or of an opportunity for taking a sample, it is possible to give at first an exposure that is decidedly insufficient and, after curtailed de velopment, estimate from the character of the image the supplementary exposure required, which supplementary exposure can, furthermore, be given in several stages, each time after partial development of the preceding impression (V. B. Sease, 1931).
442. Reversal by Blackening the Residual Silver Bromide. Reversal by a "residue" method has the disadvantage that, as a rule, it gives, with ordinary plates and films, fogged images. That is because there is more silver bromide than is necessary to give a satisfactory negative and positive.
If, in order to obtain the final positive, all the silver bromide which has not been used to produce the first negative is developed, pure whites cannot be obtained unless in the image of the whites all the silver halide has been brought into developable condition during the first exposure and then developed. This expo sure must therefore be longer as the sensitive emulsion has a greater weight of silver bromide per unit of surface. With a given emulsion the maximum speed can be obtained only by reduc ing the emulsion thickness to the minimum necessary to form the image of a normal subject. This method is therefore in current use only for thin specially prepared emulsions (plates and films with a mosaic trichrome screen for direct colour photography ; reversible film for am ateur cinematography ; papers used in automatic portrait Contrary to the instruc tions given in the case of negatives fixed after development, the exposure must be regu lated only according to the brightness of those high-lights of the subject that are to be repre sented in the final image by the lightest grey that can be differentiated from white.