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204 HISTORY OF COLOR PHOTOGRAPHY
not seem to be an extremely difficult matter to prepare them one's self. Consider the screen to be used to make the magenta printer. It is desired to opaque out the red and the blue elements, and to pass completely the green elements. This may be done by making a contact print of the taking screen upon a panchromatic emulsion, through a red and a blue filter. The red filter would opaque out the areas immediately behind the red elements, and the blue filter would accomplish the same result behind the blue elements. Development should be carried out to gamma infinity, so that a density of at least 2.5, and preferably one of 3.5, will be obtained. The spaces behind the green dots should remain completely transparent. Therefore a process pan emulsion should be used. The screen and the negative should face each other, hence the right and left positions in the screen must be opposite to those in the negative. The procedure outlined in the preceding paragraph would not give this reversal of direction.
One method of accomplishing it would be to make master negatives of the taking screen by contact, through the three filters. To make the block-out screen for the magenta printer, the master negative is made through a green filter. If a positive is made from this, also by contact, the areas corresponding to the red and blue elements in the taking screen will be opaque; only the green will be transparent. The other two screens are made in an analogous manner. G. B. Harrison (Eng. P. 434434) disclosed a system of block-out screens, but these had been previously described by C. L. A. Brasseur and S. P. Sampolo (Eng. P. 8390/96; U.S.P. 571314), C. L. A. Brasseur (Eng. P. 15185/05, 710/07; U.S.P. 897815), and G. S. Whitfield (Eng. P. 167793).
Two types of duplication are possible from screen plates. Reproductions can be made upon a similar screen material, or separations can be made by means of filters, and the separations then used for the making of duplicate prints by any of the positive procedures. A great deal of study has been made with regard to both of these types of duplication. Because of the impure transmissions and absorptions of the screen colors, it was early determined that only sharp-cutting filters could be used (C. L. A. Brasseur; Eng. P. 4932/07 ; Ger. P. 214323 and 219821; Fr. P. 367834; U.S.P. 1163207). When printing one screen upon another, an interference pattern known as moire is formed. The intensity and visibility of this could be varied somewhat by angling the two screens relative to each other, a fact disclosed by A. Lehner (U.S.P. 1113359; Ger. P. 221916). This phenomenon is well known in the photomechanical industry, where the halftone negatives are screened at different angles to overcome moire.
The general problem of reproduction from screen plates, was studied by C. E. K. Mees (Brit. J. Phot., Vol. 54 (1907), Col. Supp., p. 49). He pointed out that when one screen plate is printed upon another, color dilution or degradation takes place. These findings were corroborated by C. Welborne Piper (Brit. J. Phot., Vol. 54 (1907), Col. Supp. p. 81; and by E. Stenger and F. Leiber (Brit. J. Phot., Vol. 55 (1908), Col. Supp., p. 30, 69). More recently