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524 History of Three-Color Photography
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G. Drake-Brockman79 preferred the following
Potassium dichromate 5 g.
Sulfuric acid, cone 10 ccs.
Water 1000 ccs.
This was also commended by R. Xamias.80 The only disadvantage being that it requires rather longer washing for removal than the permanganate ; but it can not give rise to black spots. F. Stephan-1 suggested a mixture of:
Potassium permanganate 2 g.
Alum, saturated solution 1000 ccs.
Which not only reversed, but hardened the gelatin; a subsequent bath of bisulfite was used.
R. Chaboseaus patented a mixture of permanganate 2, alkaline bisulfate 18, potash alum 15 parts, which was called "Inversol," and which was to be dissolved when required in a liter of water. R. Kriigener83 patented the same combination.
The Second Development. — After the primary image has been dissolved, and this should always be done in the darkroom, the plate must be washed to remove the last traces of the reversing solution. The makers state that this need only be for 30 to 40 seconds, but it is preferable to wash for at least a minute or two, and this can be carried out in white
ight.
The condition of the plate at this moment is that the primary image produced by the development of the silver halide grains, exposed in the camera, has been dissolved by the reversing solution. There is then left a gelatin film containing unexposed and undeveloped silver bromide, where the light did not act in the camera, plus those parts of the film, which have been freed from metallic silver, where the colors of the screen elements are visible. The silver salt thus unreduced is converted to metallic silver by exposing the plate to white light with subsequent development. For this second development, the makers recommended the use of :
Amidol 5 g.
Sodium sulfite, anhvdrous 15 g.
Water '. 1000 ccs.
Too close adherence to this formula is of no moment, in fact there is no reason why the solution used for the primary development should not be used, nor any particular solution that appeals to the operator. The only requirement would seem to be that the developer should not stain.
There is one point, on which little stress has been laid, but which is important, and that is one well known to every photographer. By extreme overexposure it is possible to convert the silver halides into such a condition that they will not give a vigorous image on development, due to reversal or solarization. It may, therefore, happen that too long expo