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632 History of Three-Color Photography
from alternating records by a discriminative or selecting printing machine, in which optical printing was resorted to, that is to say, the production of the image by means of a lens, in contra-distinction to printing by contact ; no particular methods of obtaining the colors are claimed. F. J. Norman47 and others proposed to make two positives from the negatives and select one of them for the final support. The image was treated with a chromium, ferricyanide or cobalt salt to obtain the blue image. This was then coated with gelatin, sensitized with dichromate, exposed through the other positive and dyed with the complementary color. The process was applicable to three-color work, and the imbibition process was suggested for obtaining the three subtractive colors.
A remarkable coincidence occurs in the patents taken out by F. W. Donisthorpe48 and W. F. Fox49 as not only is the process the same, but in some cases the phraseology bears a resemblance. These processes may be summed up by saying that a positive from one color record negative is superposed on the negative of the other color record and a positive made from the two. The image thus obtained was toned a blue-green by means of vanadium, uranium or other compound, that will so harden the gelatin, where the toning takes place, that it will no longer absorb a dye solution. The positive thus obtained was then immersed in a bath of the complementary color, and the compound positive obtained bound up or otherwise combined with a black and white positive. Variations of the toning baths and procedure were given, but the main principle remains the same.
E. Sommavilla50 also proposed to combine negatives and positives in a somewhat similar manner to the above. E. Sommerfeldt51 patented the production of pictures by the use of two filters only. The positives from the same being stained up in the complementary colors ; the third impression being obtained by a combination of the two negatives. E. Albert52 had patented the combination of the three constituent negatives with a positive from a black key plate as a printing matrix. H. N. Hyde53 also patented the superposition of negatives and positives to obtain correct color values.
S. J. Cox54 proposed to print from one record negative and develop with a developer of non-tanning character, such as amidol or ferrous oxalate, the images being toned blue with ferrocyanide and fixed. The film was then sensitized with 5 per cent solution of dichromate, exposed under a positive in appropriate register, and the film, after washing, toned with pinatype red, or other suitable dye, which only takes on the unexposed gelatin. For three-color work the dyes should be mordanted with cupric sulfate, coated with another dichromated film, again printed and dyed up with a yellow dye of like character.
F. E. Ives55 would adopt the same process almost, that is, toning the first positive with a ferrocyanide blue, then dichromating, exposing, wash