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484 History of Three-Color Photography
filter is pointed out and this the inventor would obviate by coating both viewing and taking screens with a thin gelatinous film stained to the correct color.
J. H. Powrie48 also used the dichromate process, but preferred fish glue as the colloid, though gelatin and albumen were disclosed. The dichromated fish glue was coated on glass, whirled so as to obtain a thin and even coating, and rapidly dried by heat, then exposed under a black and white screen, the transparent lines of which were half the width of the black. Development was effected with cold water, and the insolubilized lines dyed up. It is here that Powrie utilized what was at that time a new principle, namely the application of an acid dye and then mordanting with a basic.49 For instance, aurophenin, an acid yellow dye was first applied and absorbed by the insolubilized colloid, then a basic dye, such as brilliant green, for the green, safranin for the red, or thiazin red might be used for the latter. The acid dyes, in consequence of the extremely thin film, gave but a faint coloration, but the basic being precipitated or mordanted, gave quite a deep color, which could not be obtained by a single color (see also Bawtree, p. 446). This process has also the advantage that the dye compound is insoluble, there being no tendency for it to bleed, and moreover, it enables the necessary absorptions to be obtained much more readily. The dyed lines were insolubilized further by treatment with tannin or formaldehyde, and the former would again tend to stabilize the dye, as tannin is well known as a mordant for basic dyes.
The surface was again coated with dichromated colloid and exposed under the matrix screen so that the previously obtained lines were covered by the opaque lines. This was effected in a very ingenious manner, for the matrix screen was shifted till the colored lines suddenly disappeared. No care was taken to see that the edge of the matrix screen was contiguous to the colored line, merely that the latter was covered, thus the lines might and would not exactly juxtapose. As a matter of fact the finished screen showed red, blue, green, blue lines and the width of the two blue lines was not the same. Powrie disclosed the use of Victoria green, emerald green and auramin in acetic solution for the green; thiazin red R and safranin for the red, and acid blue, soluble blue, methyl blue or methyl violet for the blue lines, and obviously others could be used ; violamin R being an excellent mordant for the blue also. The inventor admitted knowledge of du Hauron's process. Obviously it is possible to make mosaic screens by this process, as was subsequently done by the inventor, by placing the matrix screen in the second exposure at right angles to the first impressed lines. It was also pointed out that the colors being insoluble there was no necessity for an insulating varnish. Further notes on this process will be found on page 574.
C. L. Finlay50 also used dichromated colloids and employed as matrix screen, one with circular dots. Stripping or transfer paper was coated