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10 History of Three-Color Photography
not be used as printing inks, one as like spectroscopically as possible should be taken. This last requirement is obvious when it is considered that the printing ink must reflect the color rays which were absorbed by the color-sensitive plate in question, or inversely ought not to reflect the colors which were absorbed by the plate for these particular colors. If we assume a plate dyed with eosin, which by the interposition of a suitable medium (certain chromium glasses are suitable for this, also films dyed with methyl-eosin picrate) before the objective, is only affected by those rays which silver eoside absorbs, the green and the yellow-green would have the strongest action, and would give a plate which, when printed by collotype, must be printed with an ink that does not reflect the said rays. This is eosin itself. The same occurs with cyanin, which is used as optical sensitizer for plates obtained behind the red medium (ruby glass). This gives a collotype plate for an ink which reflects all those rays except those absorbed by the cyanin. This is cyanin itself. The fact that cyanin and other dyes do not actually sensitize for the rays absorbed by the pure hues, but for the neighboring ones lying more towards the red is no objection, as the ink by admixture with media of stronger refractive index (oil, etc.,) can be displaced more towards red and with an approximation so close that the difference is not appreciable to our eyes. Following out this train of thought, we come to the conclusion that every plate, which has been rendered sensitive with any given sensitizer, must be reproduced with the color which was used as the optical sensitizer. Now the light-sensitive cyanin, and still less bromide of silver (which on account of its absorption acts as an optical sensitizer) can not be used as printing inks. But other colored substances can be used which show an equal or similar absorption band."
With reference to the theory outlined above von Hubl19 pointed out that this must not be taken too literally, nor must it be assumed that the sensitizers, for instance, cyanin, eosin and a yellow pigment should be the actual printing colors, for cyanin and a yellow can never make green. The silver bromide is not sensitized for those rays which the dyes absorb, but for those which correspond to the absorption of the stained silver salt. Silver bromide, dyed with yellowish eosin, is a violet-red color, and the ink used should be of a spectroscopically similar color absorption, thus like rose Bengal. Vogel's theory is, as he says, not theoretically strictly correct ; but of practical value and characterizes in an easily grasped form for the laity the connection between the printing inks and the plate sensitizing.
With regard to the printing inks they should, when printed one over the other in saturated tones, give black ; and when in half tones give grey, and their absorption bands should be approximately of the same breadth. Transparency is naturally requisite as they are printed in superposition, and if this requirement is not fulfilled the values of the underlying colors