The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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Photography in Natural Colours
condensers, sending a beam of parallel light on to the picture whence it is reflected and taken up by a projecting lens and thrown on the screen.
The Three Colour-sensation process. This is based upon the theory of colour sensation first promulgated by Young, Helm-holtz, and Clerk Maxwell, which assumes that, although we have many spectrum tints, yet our eyes are provided with three sets of nerve fibrils, one conveying the sensation of red, another of blue, and another of green, the other and inter-mediate colours being excited by a mixture of the red, green, and blue in varying proportions. An early suggestion was made by Collen, in 1865, and practically tried by L. Ducos du Hauron and Cros ; also later by Ives, Vogel, Kurtz, and others. The subject may well be divided into two heads, first, the production of lantern slides or transparencies, and, secondly, the production of prints. For the production of lantern slides the process is by no means difficult. We require three nega-tives, a representing the red sensation, b the green sensation, and c the blue sensation. For the red sensation a red sensitive orthochromatic plate should be used, or an ordinary plate which has been sensitised by a solution of cyanine, one-third of a grain in an ounce of absolute alcohol; the plate should be flowed over with this, dried in absolute darkness, then immersed in distilled water for two or three minutes, and again dried in the dark. For the coloured screen to cut off the blue rays a piece of orange-red glass should be used. For the green sensation a commercial isochromatic plate should be used, and two thick-nesses of chromium green glass. For the blue sensation an ordinary plate with two thicknesses of cobalt blue glass should be used. From these negatives, lantern slides should be made in the ordinary way, and projected by means of a triple lantern. The transparency representing the red sensation should be pro-jected through the glass used to take it, that of the green sensa-tion through one thickness of the green glass, and that of the blue sensation through one thickness of the blue-violet glass. If the images are accurately superimposed, the result will be a reproduction of the objects in their colours. Mr. Ives' photo-chromoscope, or kromskop, is a device for simultaneously viewing the three positives, each illuminated by its appropriate colour. The group of three positives is called a chromogramme or