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History of Three-Color Photography
would be four elementary colors, blue and orange as one pair, red and green as the other, each pair being complementary and forming white.
Screen-Plates with Resists. — It is frequently easier to prevent the deposition or absorption of a color, than it is to remove the same after it has been absorbed and, therefore, the use of resists to protect the supports of possible screen-plates has received considerable attention.
A. Baumgartner89 first printed a red grain on gelatin-coated celluloid in an oil color by lithography. Then dipped it into a blue-green dye solution which only took on the unoiled parts. A grain of yellow might then be printed and, as there would be partial covering of the red and blue, orange and green would be formed. Or celluloid might be printed in red and yellow, so that they overlapped, then a coat of dichromated gelatin dyed blue might be applied and exposed through the back, when the light would not act through the red and yellow and on washing the blue would
sr jr t* r
Fig. 131. Du Hauron & Bercegol's E.P. 194, 1907.
be left in full depth on the unprotected parts, and as a little action would take place through the yellow, some of the blue would adhere and form green.
L. Ducos du Hauron and R. Bercegol90 used a greasy resist of one or more colors applied with a ruling pen. Thus, a sheet of glass or celluloid was coated with gelatin, a coat of varnish colored green applied and lines drawn in parallel through the varnish and the sheet dipped into an aqueous solution of orange dye, which would soak into the gelatin through the fine lines. A third color was provided by coating again with varnish, colorless and impermeable to water, and fresh lines were drawn through this, rather deeper than the first ones, so as to expose the lower layers of gelatin that the first lines did not reach. These lines might be parallel, oblique or at right angles to the first. The sheet was then to be dipped into a violet bath, which would take on the gelatin. Celluloid superficially