In addition to these processes, colour images have been made by using for dyes the mordanting power of chemical compounds and especially the ferrocyanides of the metals. The ferrocyanides of uranium, copper, vanadium, and silver, and the iodide or sulpho cyanide of silver mordant basic dyes very strongly. From the negatives silver images are therefore prepared and transformed into these compounds. The blue image is commonly composed of iron ferrocyanide itself, this being a very suitable shade of blue for colour work. Yellow and red images are obtained by mordant ing basic dyes, such as auramine and safranine, on to transparent deposits of metallic compounds. The three images are then superposed as usual.
In one group of processes the dye images themselves are trans ferred from the printing plates, which are formed of colourless reliefs, these being stained up and then placed in contact with gelatine so that the dye migrates and forms an image in the gela tine. Such processes are known as "imbibition" processes. The transfer plates may be made in several ways : thus, bichromated gelatine on film may be exposed through the back and developed with hot water, or a silver image produced by exposure through the back of an ordinary emulsion coated film may be caused to harden the gelatine either during development, since some devel opers harden the gelatine contiguous to the image, or by treatment with a bath similar to that used in the ozobrome process already referred to. After treatment, a relief can be obtained by washing off the unhardened gelatine with hot water, and after removal of the silver a colourless relief is obtained. In a process known as the "Pinatype" process, bichromated gelatine is exposed under a positive and is then treated with dye which penetrates only the soft gelatine, the hard gelatine remaining uncoloured. This dye can then be transferred by imbibition to a layer of soft gelatine coated on paper, and the three images can be transferred in turn to produce a colour picture.
In a modification of this, used in motion picture colour pho tography, the local hardening is produced not by the light ex posure of a bichromated surface but by the formation of a silver image and its treatment with a bleach bath which hardens the gelatine in contact with the developed silver.
There are numerous modifications of these printing processes all of which can be employed with more or less success but all of which require a considerable amount of skill if really good results are to be obtained.
The largest field for the subtractive process of colour photog raphy is in connection with the use of the printing press. The three, negatives are used for the production of printing plates, usually by the half-tone process, and prints from the three plates, frequently with a fourth black plate added, are superimposed, the plates being printed in red, blue, and yellow inks which are ap proximately complementary to the colour of the taking filters.
This three or four colour half-tone process is very widely used for the preparation of illustrations and is by far the greatest applica tion of colour photography. (See PHOTO-ENGRAVING.) Screen Plate Processes.—In his book, Ducos du Hauron sug gested a modification of the additive process which has since become of practical importance and is known as the "screen plate" process. He suggested that the surface of the glass plate or film might be covered with tiny filters—red, green, and blue— the sensitive emulsion being coated on top of these and photo graphed through the filters.
A modification of this method was suggested by J. Joly of Dub lin and by J. W. MacDonough of Chicago in 1892 ; the colour ele ments were on a glass plate which was pressed during exposure into close contact with a separate panchromatic plate after de velopment of the negative; a positive in silver was made in the usual way and viewed in contact with a special viewing screen, in which the colours approximate those required by theory. This method is employed in the Finlay process much used in recent years, particularly for work in the studio and in the field for making colour originals from which three-colour printing plates can be made. Any number of duplicate colour transparencies can be made from one original negative.
In 1907 Messrs. Lumiere and Co., of Lyons, brought out their "Autochrome" plates in which they utilized an irregular screen composed of coloured grains. Glass plates are coated with an adhesive medium over which there is spread a mixture of starch grains, of microscopic fineness, stained violet, green, and orange, the interstices being filled in with fine carbon powder, to form a tricolour screen, dark by reflected, and of a pinkish, pearly ap pearance by transmitted, light. This is varnished and coated with a thin panchromatic emulsion of gelatino-silver bromide. The plates are exposed in the camera from the back, through the tri colour films, a special compensating orange-yellow screen being used also before or behind the lens. They are then developed as usual, producing a negative coloured image in the complementary colours, which is then reversed so as to produce a positive coloured image showing the picture in its proper colours. The results thus obtained are remarkably good and practically solve the problem of direct colour photography in a simple and fairly inexpensive manner, the results being of course confined to transparencies. In recent years other screen plate processes of this type have been introduced. They include the Agfacolor plates and film, in which stained resin grains are used instead of the starch grains of the Autochrome process. In Dufaycolor, the emulsion is coated on a screen ruled on the glass or film support to give a regular pattern.