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Principles of Three-Colour Methods 865

light, rays, grey, green, black, white and filter

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PRINCIPLES OF THREE-COLOUR METHODS 865. General Notes. The concept of Ducos du Hauron, as set out in his publications, may be summarized as follows (E. Wallon).

Any colour, natural or artificial, is a combina tion of the elementary rays contained in white light and which the spectrum shows us dis associated (§ r). To obtain an exact analysis and synthesis of a colour, it is necessary to separate all the radiations which compose it, to measure them, and to re-combine them in the same proportions. Now, these elementary rays are infinite in number, so that the double opera tion seems impossible. Experience, however, fortunately reveals to us a simplification which renders it possible. It shows us that we can first gather the elementary colours into a limited number of groups which can then be treated as indivisible elements. Thus, in analysis, it will be sufficient to take the whole of the radiations belonging to each of the groups without need to detail them. For synthesis, instead of introduc ing the radiations themselves into the combina tion, we can again take these groups, without troubling whether they occurred complete and with their normal composition in the complex colour that we wish to reproduce.

The number of these artificial groups may be very limited. It is sufficient to form three, suitably selected, in order that the double operation of analysis and synthesis may be fully satisfactory in practice.

866. Trichromatic Selection. If a given multi colour object illuminated by white light is photographed three times, on plates of appro priate sensitiveness, and through filters which are respectively blue-violet, green, and red, and divide about equally the visible radiations of the normal spectrum, it will be found that the three negatives obtained differ very consider ably. While white, neutral grey, and black are rendered to the same extent in all three negatives when the exposures have been correctly pro portioned, colours will be rendered very differ ently in them.

A pure saturated yellow for instance, which reflects nearly all the red and green rays, but absorbs the blue rays of incident light (§ 5), will be photographed as a very light grey would be through the red or green filters, and like a black through the blue filter. A vermilion,

which reflects the major part of the red rays and absorbs the green and blue rays, will be rendered like a light grey through the red filter, and like a black through the two other filters. A yellow ish-green, which reflects most of the green rays and a small fraction of the red rays, but absorbs the blue rays almost completely, will be rendered as a rather light grey through the green filter, like a medium grey through the red filter, and like black through the blue filter.

The differences between the negatives will remain, though less marked, for light, mixed, and dark colours (§ 8). Thus, for instance, a sky-blue, which reflects all rays but with the predominance of blue, will be photographed nearly like white through the blue filter and like a light grey through the two other filters. A myrtle green, which reflects some few green rays and absorbs all the others, will be rendered as a dark grey through the green filter and like black through the other two filters.

867. Additive Synthesis. Let us suppose that three transparencies, printed from the negatives described above, are each placed in a projection lantern, the three lanterns being adjusted so that the three images are superimposed on the screen. Let us place in each of the three beams of light the filter used in taking the correspond ing negative, and let us adjust the intensities of the three beams so that white light is produced. on the screen in the absence of the three trans parencies, or, if not white, at least a light without any predominant hue.

In the portions corresponding with the pure whites of the subject, the three coloured beams (transmitted at their maximum intensity by the transparencies which are quite transparent in the image of these whites) will form white light on the screen. The image of a neutral grey will be of equal density in all three trans parencies ; the three beams, weakened propor tionally, will still produce a white light, but one of reduced strength, giving the sensation of a more or less dark grey by comparison with the pure whites. A black in the model will be rendered as black in the three transparencies, which will not allow any light to pass to the screen and will thus give the impression of black.

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