If the positives are to be made by the iodide or mordanting process, it can be easily reversed, if necessary, in the stripping process.

If the dialyte or semi-dialyte systems be adopted, the printing colors are merely those of the normal three-color process, and these have already been dealt with. But with a two-color system we have to adopt a compromise: practically we must use a more orange red and a more greenish blue than the standard three colors, but any of the actual printing methods may be used.

For the diachrome process or those in which basic dyes are used, rhodamin 6GF or 12GF may be used, but as there may be difficulty in obtaining these, one can use fuchsin and auramin for the red and malachite green and auramin for the blue-green. If the relief or other methods which require acid dyes are used, then a mixture of naphthol green, acid green and naphthol yellow may be used for the green and a mixture of the acid reds with naphthol yellow for the red picture. Exact ratios of the dyes cannot be given, but a few trials will soon determine the best composition to use; in all cases the results should be examined by artificial light, for, as already stated, two-color pictures are only satisfactory when thus viewed. In print making, the colors should not be too deep, as the light has to pass through the film and be reflected from the white support. This obviates one trouble that is frequently met in transparency making, which is that the deep shadows are sometimes either violet or red and not black. This is chiefly due to the faulty transmissions of the dyes; if the shadows are violet, the red element probably wants a little more yellow in it, while if they are red, increase of the naphthol green will cut this out.

The Kodachrome process was introduced by the Eastman Kodak Co. in 1915, and is a two-color process based on the selective action of dyes for hardened and un-hardened gelatine. Like all the other two-color processes it fails in the correct rendering of the blues, violets, magentas and purples; but for flesh tints, reds, oranges, greens, greys and blacks it is excellent, and some exquisite results can be obtained if the above limitations are borne in mind.

Briefly, the method of procedure is to expose two panchromatic plates behind suitable color filters and, after development, to so treat the plates that the gelatine is hardened in situ with the metallic silver and, after fixing and drying, stain up with the special dyes and superimpose. The best effects are obtainable with artificial light as the illuminant.

The working details follow; the two panchromatic plates can be exposed simultaneously, which obviously requires a special camera, or in succession. A special lighting system was devised by the Eastman Kodak Co. for portraiture, though naturally daylight can be used. The plates should be developed with metol-hydrochinon and the negatives should be rather of a soft character, which is obtainable as usual by shortened duration of development; over-exposure should be avoided as far as possible.

There are two courses now open to the worker: either to fix the negatives in the usual way and make duplicate negatives therefrom, or the less tedious plan of converting the original negatives into the dyed positives. For this latter system the negatives should be washed after development for about ten minutes and then bleached in the following:

A. Potassium ferricyanide 37.5 g.

Potassium bromide 56.25 g.

Potassium bichromate 37.5 g Glacial acetic acid 10 ccm.

Water 1000 ccm.

B. Potassium alum, 5 per cent solution.

For use, mix in equal volumes. Care must be taken to cover the plate with one sweep of the bath and use not less than 250 ccm for 500 square centimeters. The bleaching will take approximately four minutes, and the action should be allowed to continue for a short time after all the black silver has disappeared, it being converted into a brown image. The plate should then be washed for ten minutes in running water and fixed in a bath that does not contain alum, such as:

A. Hypo 250 g Water 1000 ccm.

B. Sodium bisulphite 400 g Water 1000 ccm.

For use, mix 1 part of B with 20 parts of A. This may be used repeatedly but not for more than 3400 square centimeters per liter (2000 square inches per gallon).

At this stage the plates look perfectly transparent with no sign of an image, and should be washed for twenty minutes and then immersed in a 0.5 per cent solution of ammonia and the dish well rocked for three minutes, and again washed for another five minutes. Surface moisture should be removed with a soft squeegee or by dabbing the plate with a pad of fluffless cloth, and the back must be dried. Drying must be uniform and the best plan is to dry before an electric fan. If the plates dry in patches, these will show in the finished picture. The drier the film the cleaner the high-lights, and the plates may be dried in an oven or left for at least three hours after drying in the usual way. This latter plan requires longer development, and the longer they are left after drying and before staining up the greater the contrasts in the resultant picture, so that if uniformity is required constant conditions must be observed.

Special dyes are issued, and a 1.2 per cent solution of the red dye and a three per cent solution of the green dye should be prepared. It is advisable to dissolve the dyes in a little hot water and filter the solutions through linen after dilution to bulk; distilled water should be used. The complementary colors are used, of course; that is, the negative taken through the red filter is dyed green, and that of the green filter dyed red. The dye solutions may be used repeatedly but should be kept up to strength by addition of fresh dye from time to time. As soon as the plates are immersed in the dye solutions the surfaces should be gently rubbed with a tuft of absorbent cotton to remove any air-bells. The average time of dyeing is about three minutes, but the progress may be watched by rinsing the plates with water, although it is not advisable to resort to this too often. Naturally they must be examined by the light by which they are subsequently to be viewed. If the composite picture shows a predominance of one color, the complementary plate should be more deeply stained. As soon as the plates are considered sufficiently stained, rinse them in a 1 per cent solution of glacial acetic acid, and wipe the surface of the film as already advised. It is important that the plates should dye quickly; the green plate dyes more slowly than the red, but even so should not take more than ten minutes; if it does the temperature of the dye baths should be slightly raised. Slow dyeing is also caused by insufficient bromide in the developer and overexposure.

To superimpose the plates, the green image should be placed on top of the red image and shifted about until perfect registration is obtained. Then the sides of the plates should be clipped together by large metal clips and short paper clips applied and these allowed to dry. If a mask is to be used it should be now applied on top of the green plate under the cover glass and the three plates bound up with long binding strips. Retouching may be effected with the dye solutions thickened with gum arabic solution.

If the duplicate negative method is adopted, and it is advantageous for several copies, then the original negatives are fixed, washed and dried in the usual way and from these transparencies made, and duplicate negatives; the best plates for both purposes being those of the character of the Seed 23. Both plates should be exposed at the same time at the same distance from the light, and developed together, aiming at fully exposed, soft, delicate negatives with all the detail of the original negatives; the further treatment is precisely as has been detailed above for the original negatives.