This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
The Principle Involved. In making an Auto-chrome picture the principle involved is as follows: The plate is put into the plate-holder the reverse way, so that the glass side shall be toward the lens when in the camera. This is necessary in order that the light shall first pass through the screen or starch grains before reaching the sensitive film of the plate. After exposure the plate is developed, and then immediately reversed, or converted from a negative into a positive. All the developed negative image being dissolved away, the remaining unaffected silver is re-developed and so forms the positive picture.
1033. On exposing the plate in the camera, all the red objects in the scene before it affect only the portions of the plate covered by the red starch grains, and the green and violet objects and tints of color only affect the sensitive film which lies over the starch grains of corresponding colors, so that when the plate is developed, these minute portions of the emulsion produce a silver deposit, which by transmitted light completely obliterates the colors which were photographed, leaving only the colors complementary to them. Thus, if this negative image is now fixed without any further treatment, all red and yellow objects will appear blue-green, green objects will appear violet, and blue objects appear yellow.
1034. Instead of fixing, however, the opaque, deposited silver particles are dissolved out of the film, leaving the required colored particles clear and bright. The remaining undeveloped emulsion is then exposed to daylight and re-developed, so that the silver, which would in the ordinary way be dissolved out in the fixing bath, is now deposited to form the picture by covering up the grains of color which is complementary to the colors of the original; and the result is a picture in natural colors.
1035. This, briefly, is an outline of what happens in the making of an Autochrome, but we will now go fully into all the details of the various stages of the work, so that if carefully followed out, failure will be impossible.
1036. In the first place it should be observed that owing to the fact that it seems to be impossible to make an emulsion which is not more sensitive to blue than any ther color, it is necessary with Autochrome plates to use Lumiere's Special Absorption Filter to correct the blue rays. This filter must be fixed onto the camera, either in front or behind the lens, and a carrier for the purpose must be provided. When using a behind or between-the-lens shutter, it is advisable to arrange the screen carrier for fixing onto the hood of the lens; otherwise it may be fixed behind the lens inside the camera. It should be noted, however, that in the latter position it becomes an additional element to the lens, and with some varieties of lenses it will have the effect of slightly increasing the focal-length. This is not so when the screen is in front of the lens, but in this case it will be necessary to reverse the focusingscreen of the camera, so that the ground-glass side is behind, in order to allow for the reversion of the plate in the plate holder.
1037. In either case it is a wise plan to finally focus with the screen in position, so that any discrepancy may be adjusted. Wherever the light filter is placed, great care must be taken to prevent any trace of light reaching the plate, except through the filter, or incorrect color rendering will result. Although the absorption filters supplied are especially adjusted for these Autochrome plates, satisfactory ones can be made by dyeing up a fixed out transparency plate in filter yellow K, to which has been added a trace of bluish scarlet. It is also advisable for landscape work to further slightly dye one-half of a filter with a green, such as Naphthol green, or brilliant green, as is used for wool. This, if used for the sky, will give a much truer rendering of the blue than is possible with a plain tint screen, because the sky is almost inevitably over-exposed.
1038 The Dark-Room. - The next consideration is the dark-room. It must be remembered that the emulsion of the Autochrome plate is sensitive to all the colors of the visible spectrum, and that, therefore, the ordinary darkroom illumination will be useless. Indeed it is far better to get into the habit of working these plates in total darkness. But if any light is used at all, it should be one of a very deep green color, and even then the direct rays of this, the least actinic color, must not be allowed to fall upon the plate.
1039. Having carefully excluded any possible trace of white light which may come from under the door or elsewhere, take a plate out of the box and, with a piece of linen rag, carefully clean the glass side and put it into the plate-holder, glass side downward. Then place on top of the plate one of the black backing cards which are supplied with the plates, with the black surface toward the film. A better plan is to have the cards covered with black velvet for this purpose, as much cleaner results are obtained than when the black cards alone are used. If a camera of a larger size than the Autochrome plate is employed, the backing cards may advantageously be hinged on to the plate-holder, by attaching a strip of gummed paper along one side. This will avoid getting the cards mislaid in the dark-room, and, what is more important, will insure against forgetting to use the backing, which is highly essential.