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630 History of Three-Color Photography
Or if this is too yellow, then 57 should be adopted. For the red, No. 23, E red, or 23 A, E **ed light, or as alternative No. 27 A, stage red light, can be used.
One can easily match these up by staining gelatinized glasses, varying the ratios of the dyes, till the desired shade is obtained when the films are dry, and they must be matched by artificial light.
Using the spectrum as a standard, it is only necessary to project it on to a white screen, and then match the color by a measured admixture of the dyes chosen. It will be noted that it is recommended to project the colors, and this is important as they are by no means the same when examined by daylight, and a mixture, correct by daylight, may look quite wrong when projected by the comparatively yellow arc. It would be possible to use a projected spectrum and the apparatus for this can be rigged up at very little cost, using a liquid cell prism, a few cheap spectacle lenses, and a slit made from a line ruled on a sheet of glass coated with black varnish. A short spectrum should be first obtained, three inches is quite long enough, and either the filter itself used, or a template cut so as to remove the colors absorbed by the filter. Then the remainder can be projected by a second lens into a color patch, which can be matched. One light only need be used, if the beam reflected from the front face of the prism be deflected on to the screen, side by side with the spectrum patch, by the aid of a mirror. The adoption of some such plan will save hours of rule-of-thumb work with positives, though probably the latter is the method usually adopted.
One of the commonest errors in subtractive pictures is the failure to obtain blacks, they being either a deep violet or red. The former is due to the transmission of the violet by the red dye, as too often a magenta type of transmission is used. While the redness is caused by the use of a green dye that transmits the deep red, and nearly all green dyes, with the exception of the naphthol greens, transmit a band, more or less bright, in the red. Frequently the violet can be eliminated by the use of a yellow dye, but modification of the green to absorb the red band is not possible by any addition, except that outlined above.
Naturally one must use dyes that are compatible, that is to say, that will not mutually precipitate. Their stability to light is of less moment, taking into consideration the short time they are actually exposed during projection. Another method of obtaining the desired result is to utilize the principle of the precipitation of basic by acid dyes, and one can first stain in an acid dye, and then pass the print through a basic dye of the modifying color, when the latter will be precipitated on the former. But this involves another operation, which should be avoided if possible, and the final dye compound may be less transparent than is desirable.
Occasionally a little trouble is met with in the differing absorption of the dyes, that is to say, if we are using a red and a yellow dye, the one