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Screen-Plate Patents 485
with soft gelatin, holding in suspension a silver halide and when dry was printed in one of the three colors, and, after moving the position of the matrix, another color was printed. The coating was now transferred to its support, and the temporary support removed. Hot water being used for development the soluble gelatin above each color patch was dissolved, the progress being visible by the washing away of the silver halide, which also prevented the too great swelling of the gelatin, this being the principle utilized by Lumieres. Two colors were printed as circles, the red and green, and the interspaces stained up in blue. Such a screen could be coated with panchromatic emulsion, or used as a separate screen.
O. S. Dawson and C. L. Finlay51 proposed an improvement on the above, in that the matrix screen was prepared by the use of the ordinary half-tone screen with a circular diaphragm, circles being thus obtained on a plate, occupying about one-third of the area. From this a positive was made and used as a matrix for printing on to dichromated colloid. After dyeing a second sensitizing was effected, the matrix shifted and a second series of circles printed and stained, the third color being obtained by dyeing the interspaces. O. S. and H. E. Dawson52 adopted the same method, only varying the shape of the elements, one being a circle, another an oval and the interspaces being of diabolos form.
C. E. K. Mees and Wratten & Wainwright53 patented the production of a screen composed of a single coating, differentially stained up, and which could be made in one operation. One method of carrying this into effect was to use a colloid, sensitized with dichromate and dyed up, or after-dyed with, for instance, patent blue. This was exposed under a matrix of graduated opacity and conveniently composed of lines in sets, each set comprising a black or opaque line, a semi-opaque and a clear line. This might be made in various ways. After printing under this matrix the plate was washed till the dye was completely removed from under the opaque line, or where it had not been exposed, and partially washed out from those lines of semi-opacity. The result would be a deep blue line, one more bluish-green, partially washed out and the third colorless. The plate was then to be soaked in a yellow dye, that would only take in soft gelatin, as a result the blue line of hardened colloid would be unaffected, the half-hard and half-soft line would be pure green, owing to the addition of the yellow on the bluish-green, and the clear colloid would be yellow. By subsequently dyeing with a red dye that only takes on soft gelatin, this yellow line would be turned scarlet.
F. Faupel54 whilst using dichromated colloids, did not develop the same but used their differential absorption for different dyes. Azorubin and crystal ponceau 6R were used for the hardened colloid, and methyl blue, brilliant wool blue G and brilliant azurin for the soft. The two-color screen thus obtained was again sensitized and exposed under a matrix with the lines running at an angle to the first and the plate was immersed in a