COLOR PHOTOGRAPHY. The reproduc tion by photography of natural objects in their own colors. There is no means known at present by which, using ordinary photographic processes, thk is possible. When a photograph is taken with a camera and a sensitive plate, the devel oped negative shows an image of the object in various shades of gray. which depend upon the sensitiveness of the photographic plate to the ether-naves characteristic of the colors of the natural object. It is possible so to stain a pho tographic plate that it is more or less sensitive to all colors: but the developed negative is always gray, except possibly for certain accidental colors which have not the faintest connection with those of the object photographed. To reproduce the colors, therefore, other methods are essen tial, and there are at the present time two quite distinet processes.
One of these is based upon the work and a suggestion of Prof. J. Clerk Maxwell of the University of Cambridge. He showed that if there were produced simultaneously in the normal eye three sensations—viz. definite shades of blue. green. and red—the eye could be made to perceive any desired color of the spectrum by properly adjusting the intensities of these three component sensations. Thus, if by any means e.g. by sets of mirrors—the eye can be made to see at one time three ordinary photographs of any natural object. looking at one through a piece of red glass. another through a piece of green glass, and the third through a piece of blue glass, the eye will see the object in its natural colors, pro vided the intensity of the deposit of silver on the original three negatives is so adjusted for each negative that the intensities of these red, green, and blue sensations are exactly such as to pro duce the proper color-sensations. To secure this intensity on the photographic plates, three photo graphs of the object must be taken, each through such a colored screen as will transmit enough light of all waVedengths to produce the desired result on the plate. Thus, one plate is exposed
in a camera in front of which is a screen which allows to pass a great deal of red and small amounts of yellow and green; the second is ex posed with a screen which is transparent to the green and slightly to yellow and blue; the third is exposed with a screen which is transparent to blue and slightly to green and violet. It is a question of the most careful experimenting to find what photographic plates should be used, and what colored screens give the proper inten sity with them. • There are three processes of color photography based this general idea. In the Ives process, the three photographs of identical sizes are taken simultaneously on three plates, each through its proper 'taking' screen. From these three nega tives, three positives are made by contact ; and these positives, each with its proper 'viewing' screen of pure red, green, or blue, placed in a so-called `kromskop' in such a manner that sun light is reflected through them and their screens, and all three pictures are seen superimposed ap parently on each other. In another process in vented independently by Professor Joly. of Dub lin, and Mr. McDonough, of Chicago, the three colored screens through which the photographs are taken are combined by having a series of lines of these three compound colors ruled very closely together on a piece of glass, every fourth line having the same color. A single photograph is taken through this composite screen; a positive is taken of this, and a viewing screen, consisting of a series of lines, one a pure red, the next pure green, the next pure blue, with the same spacing as in the taking screen, is superimposed on the positive, so that the colored lines come in ex actly the proper positions; and this compound plate is used as a transparency by holding it up to the light, or by looking through it at a piece of white paper which is brightly illuminated.