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456 History of Three-Color Photography
element, merely for the method of producing the same : "The colored lines can be produced either by purely mechanical means, or by photography itself. As regards the mechanical means, the method most to be recommended consists in passing the transparent tissue over three grooved cylinders, each of which, of very narrow diameter and very close to one another, receives an inkage of one of the three colors by contact of its ridges with a corresponding inking roller. As regards the use of photography itself for the formation of the colored screens, this will chiefly find its application in those cases where one requires to have a screen of extreme fineness. Preliminary to the description of this process, I ought to point out that it is a matter of absolute indifference, for the optical results obtained, whether the screens be composed of straight lines or constantly parallel, or of some geometrical divisions, provided that in a given area, the division of each of the three colors reproduces the same amount of surface for each of them. This being granted, let us assume that a dichromated colloid, colored red-orange, is spread over the tissue, and that this surface is exposed to light through the back under a very transparent screen, composed only of opaque and transparent lines, whether mechanically drawn or obtained by photographic reduction, the opaque lines having double the width of the transparent ones. The development bath will cause a series of parallel, red, lines to appear, visible with the magnifying glass, and with colorless transparent interspaces, the latter occupying two-thirds of the surface. If after having hardened this surface and rendered it impermeable by any well known means (the latest and most satisfactory is hardening with formaldehyde) the screen is coated with a second dichromated mixture, this in turn dyed green, and if this be exposed to light, not through the first line screen, but through one made in a similar manner in which the black lines are of equal width to the transparent ; and if, moreover, this screen be turned in such fashion that the lines take a direction at right angles to the red lines already formed, the light will be intercepted not only by each black line, but also where the red lines extend over the spaces between the black lines. The second development will then cause the green lines to appear, which the red lines will divide into small rectangular pieces; these pieces leave between them colorless transparent spaces of the same shape and area. Finally, if after having hardened and rendered impermeable this second coating, the screen thus transformed, is covered with a third and final dichromated mixture, dyed this time in blueviolet, and it is exposed through the back to light, but now without the interposition of any black-line screen, the light will act on the rectangular spaces not covered by the red and green lines ; these two lines act as nonactinic filters for the dichromated preparation which they cover. On development, at the same time as the red and green lines disengage themselves from the blue-violet film uniformly coated over them, one will see all the blue-violet areas show themselves in turn, and the final result will