Description of the Three-Color Process.— The ((three-color process° and the °tri-chro matic process,° are the names given to an in direct process of color photography and color pnnting which has been successfully developed vanous applications. The first modem sug gestion of such a process was made by Prof. James Clerk-Maxwell, in 1861 or before, and the principle was reinvented and elaborated by Lotus Ducos Duhauron and Charles Cros in 1869, but the results obtained experimentally were unpromising, and owing probably to this fact and also to the existence of a general preju dice against the idea of color photography by any but a °direct° process, the subject received but little further attention until F. E. Ives, in 1888, stated a new and definite principle of photographic color selection, and gave a suc cessful public demonstration at the Franklin Institute, in Philadelphia. Tlais was followed by a great revival of interest and active experi ment ,and the conamercial development of the process in various applications. The first step in this process is the production of three pho tographic negatives which constitute a record of the color values in terms of spectrum red, green and blue-violet lights, these being the only spectrum colors which will by admixture in various proportions reproduce all other colors without material degradation of purity. Such sets of negatives are made by exposing color sensitive photographic plates in the camera through selective °color screens? Formerly a special red suisitive plate was exposed through a red or orange screen, a green sensitive plate through a green screen and an ordinary plate through a blue or violet screen. Following Ives, most authorities now recommend the use of only one kind of plate, sensitive to all colors, in order to ensure uniformity of gradation and density in the three images with simultaneous development. The equalization of the ex posures, also an important element of success, is best accomplished by the use of special cameras, which form the three images simul taneously, from one point of view. Several such cameras designed by Ives make the three images, identical in size and perspective, side by side upon one plate, at one exposure, thus malting the negative process as simple as it is in monochrome photography.
From the photographic negatives thus ob tained, which record the colors only by differ ences of density and gradation in the separate images, colored pictures can be made by two synthesis methods, one °positive° and the other unegative? In the method of positive synthesis, red, green and blue-violet lights, in terms of which the three negatives have recorded all the colors of the objects photographed, are used to illuminate three positive ub/ack and white° im ages made from the negatives, and these three images are then optically blended to form a single image, in which the colors mix to repro duce to the eye die colors, form and light and shade of the objects photographed. Such optical synthesis is effected either with three magic lanterns or their optical equivalent, or with an instrument called a photochromoscope, which is used like a stereoscope. In the stereoscopic photochromoscope of Ives, the reproduction is so perfect that the objects themselves seem to stand before the eyes. In the method of negative synthesis, positive color prints are made from the three negative images, and super posed in white light, or upon a white surface. It is an interesting fact that the colors used in printing are not the red, green and blue-violet photographing colors, but their complementary colors, a peacock blue, a crimson pink and yel low, commonly but incorrectly called *blue," ((red° and yellow. This is because the printing
process makes and mixes shadows instead of lights, being complementary to the method of reproducing colors in the photochromoscope.
The negative made through a red screen inust print a positive, uncolored in its high lights and peacock-blue in its shades. The negative made through a green screen must print a posi tive, uncolored in its high lights and crimson pink in its shades. The negative made through a blue screen must print a positive, uncolored in its high lights and yellow in its shades. It is also important that these colors be perfect! transparent. When these prints are superpose equal parts of all three colors form blacks an grays, and the pure colors, red, green and blue violet, appear where yellow is superposed on crimson-pink, peacock-blue on yellow and crim son-pink on peacock-blue. Unlike the method of positive synthesis, no method of negative syn thesis fulfils all theoretical requirements in ordi nary white light, and if the process is employed to reproduce as difficult a test as the spectrum itself, it must fail to do full justice either to gradation of hues and luminosity values or else to purity of color. The degradation of purity of color resulting when the analysis perfectly differentiates all hue and luminosity values, as in the original method of Ives, is not so consider able as to appear objectionable when perfectly transparent and correct printing colors are em ployed; on the other hand colors showing diffuse absorption in the spectrum are often pretty cor rectly reproduced with an analysis in the neg ative process which, while favoring brilliancy of color in the reproduction, would fail to secure a passable representation of the spectrum. Con sequently, authorities, having different opinions as to what qualities are most desirable in a re production, must and do disagree as to what is the best principle of color selection for neg ative synthesis.
This difference of opinion is particularly justifiable in connection with the half-tone tn chromatic process, thus far the most important conunercial development of three-color photog raphy, because the subjects most often repro duced have colors showing diffuse absorption in the spectrum, and the best reasonably perma nent printing inks thus far obtainable are neither correct in hue nor of sufficient purity and trans parency to fulfil their theoretical functions.
The first half-tone trichromatic process prints were made by Ives in 1881, but the process was not developed commercially until reinvented and patented by Albert, Duhauron and Kurtz, more than 10 years after. In this process, half tone blocks are made from the three negatives, and printed in the type press, with the peacock blue, crimson-pink and yellow inks. In order to prevent the production of an offensive moire pattern, the lines of the half-tone blocks are dis posed at different and suitable angles for the dif ferent colors. Owing to the complications of the process, and the theoretical imperfections of materials and means necessarily employed in its commercial operation, the pnnting plates are more or less re-etched in parts until proofs show the desired result, but with so little work and cost altogether that the process is competing suc cessfully with chromolithography for many pur poses, and may eventually supersede it, as half tone engraving has already superseded wood engraving.