The history of three-color photography (1925)

446 History of Three-Color Photography progress of development to be better observed. Separate plates could be used, or one plate, and the individual color films being each protected with an insulating coat of Canada balsam, before recoating with gelatin. H. Schmidt17 proposed to make a silver transparency and tone as usual with cyanotype, then coat with dichromated gelatin and print under the greenfilter negative, stain with red and repeat the operation for the yellow. He claimed that the water dissociated the dichromate into chromium oxide and chromic acid, and that the dyes formed lakes with the same, obviously agreeing with Selle. A. E. Bawtree18 published a very complete paper on the preparation of colored transparencies, that, although it does not primarily deal with tri-color work, is worthy of somewhat full condensation. The colloid he used was fish glue, which whilst it had been much used in photomechanical processes, had not been much employed in color work. Like gelatin and other colloids, it possesses the property of becoming insoluble by the action of light in the presence of dichromates. The novelty of the process lies in the fact that although some dyes have but little affinity for gelatin, yet they have a strong affinity for each other, and, therefore, by first staining up, though but faintly with one dye, the subsequent application of a second dye gives a very strong colored image. It should be noted here that J. H. Powrie19 had not only used fish glue, but also the very principle that Bawtree claimed as novel, namely, the mordanting action of basic dyes for acid, and vice versa. Either of the following might be used for the sensitive coat, though Bawtree preferred the latter: A. Le Page's fish glue 500 ccs. Water 500 ccs. B. Dried albumen 60 g. Water 1000 ccs. When dissolved add Ammonium dichromate, 10 per cent sol 400 ccs. To make A put the water into a graduate and add the viscous fish glue, which should be the special clarified kind, up to the 1000 ccs. mark ; this obviates measuring the glue in a separate vessel, stir well till completely mixed and add B, filter through absorbent cotton. This solution must be kept in the dark. The preparation and the coating of this mixture must be done by artificial light. The second formula was : Fish glue 570 ccs. Water 285 ccs. Ammonium dichromate, 10 per cent sol 145 ccs. Both solutions would keep well in the dark for about 2 months, and then the sensitiveness doubled. Whilst Bawtree recommended line negatives only, there is no doubt that the process is applicable to half tone, if the film is printed through the back, and he suggested that old negatives should be freed from their