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.
By Henry J. Comley, F. R. P. S., Stroud, England.
1027. The "Autochrome" screen-plate process of color photography has been truthfully described as the greatest advance in photography since the advent of collodion emulsion; for it has given to the world the first really practical and commercial method of making camera pictures direct from nature, which faithfully reproduce all the subtle charm of her beautiful form and color. This is the solution of the great problem which every photographic scientist since the days of Niepcie and Daguerre has longed to solve, and yet it has at last been solved in the identical manner which was suggested as possible by Louis Ducros du Hauron, of Paris, forty years previously. Du Hauron, in 1868, claimed that if a photographic plate was prepared with a transparent paper screen attached, upon which either lines or dots of transparent color were closely printed, it would be possible to make pictures reproducing nature's colors direct in one operation in the camera. But, although more than one praiseworthy effort was made, both in America and Europe, to make a commercial success of a process worked out upon these lines, it has remained for fellow patriots of Du Hauron to bring it to a successful issue.
1028. In the manufacture of the Autochrome screen plate, Messrs. Lumiere have had to overcome mechanical and physical difficulties which would have caused most men to give up in despair, so that the success to which they have attained is something more than the outcome of scientific research.
Process Of Manufacture. A brief description of the manufacture of these wonderful plates will make this clear. Carefully selected sheets of glass are polished and coated with a transparent glutinous substance, upon which is dusted a mixture of colored potato starch grains, practically uniform in form and size, and of such microscopic proportions as about 20,000 to the inch. The plates are then put into a machine which presses and slightly flattens the starch grains so that the small intercesses between them are, as far as possible, filled up; the remaining spaces are then filled in with carbon black.
1030. A coating of water-proof varnish is next laid over the starch grains, and upon this is spread a panchromatic photographic emulsion of extremely fine grain, which consists probably of a mixture of gelatin, with a substance similar to collodion. The starch grains are of three colors, i. e., red-orange, blue-violet and green. These colored grains are mixed in such proportions that the mixture is of a neutral gray tint. The proportions of each color per 100 grains is as follows: green, 45; violet, 28; red, 27. These microscopic light filters do not differ materially in color from those usually employed for three-color work, and by various combinations of quantities of these colored grains, all the colors of nature are reproduced.
1031. It will be noticed that there are neither yellow nor blue grains employed, yet these two colors are most faithfully produced in a finished Autochrome picture. The combination of red and green light equals yellow, and so red and violet equal blue. White is, of course, an equal combination of the three colors, and though a pure white would seem to be impossible upon these plates, when sufficient depth of color and shadow surround a white object it appears to be a beautifully pure white by contrast with its surroundings. So that it may, with some truth, be said that the color effect presented to the eye, by these plates, is a scientific optical illusion. But whatever it may be, the results are really wonderful and beautiful, and the inventors deserve all the success to which they have attained.