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Screen-Plates — Historical and Theoretical Data 455
violet, which may be too great from the action of the two rays, then one need only lightly shade with a crayon the red and blue lines. To produce grey, the luminosity of all the lines must be reduced, and black will naturally be produced by obliteration of all three. That which may be done by an expert hand may all be done by natural forces. In fact, assume that the surface of this paper, on the side covered by the lines, is coated with a preparation which will give direct a positive print by the action of light, and that on the back, that is on the side not covered by the lines, is formed the image of the camera; the result will be that the three simple colors will be separated in passing through the paper, and each would form its positive imprint, that is to say, its impression in light on the lines of the corresponding color; and the three impressions will be formed with the same rapidity, in spite of the unequal degree of the actinism of the three simple colors, if care be taken to give each of the three lines a transparency in inverse ratio to the photogenic power of these same colors on the preparation used. This unequal transparency may be produced by dark lines previously formed on the back of the sheet, and the most simple means to obtain these lines consists in negatively sensitizing, by means of silver chloride, for instance, the back of the sheet, and exposing through the front to diffused light, so that there is formed by the unequal action in passing through the different kinds of lines, dark lines possessing the required degree of purity. To make this method of three lines practical, it would be preferable to employ as auxiliaries, the indirect processes of photography. One might have a single film, such as a sheet of mica, covered on one side with red, yellow and blue lines of an intense color, and on the other the lines of unequal opacity. Such a film would then serve to obtain other surfaces (paper, glass, etc.) brought in contact with negatives in silver bromide. Each of the negatives would then furnish in turn positives, in black or carbon, a film, glass or mica, etc., and all that would be required would be to apply one of these positives to an opaque or transparent surface, mechanically covered with red, yellow and blue lines, each corresponding in position to the lines of the film, which was used as a filter for the lines of the simple colors. The lines might be produced either by pure mechanical means, such as chromo-lithography, or by photography itself, by means of black and white screens, reduced in the camera, and through which preparations of gelatin, or gum or albumen, etc., dichromated and colored might be exposed. The three lines, instead of being printed on one sheet, might be made on three separate sheets, which might then be superposed to produce juxtaposition of the lines. The yellow lines ought to be replaced on the filter by greenish-yellow lines, as I have already pointed out."
In a later work4 occurs the following passage, which is given in its entirety, because of its historical importance, and because it conclusively proves that no patent can be valid for any particular form of screen