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Historical and Theoretical Data 17
the red and blue negatives. After explaining how the printing colors are laid down from the negatives, the conclusions arrived at were, that "considered from the point of view of the aforementioned treatment, the tricolor process by negative synthesis would appear to depend only indirectly upon the tri-color theory of vision, for the colors are approximately at least imitated independently of the mechanism of vision. It appears from the foregoing that the region which is recorded through both the green and the red filters is reproduced in the printing color of the violet record, viz., a yellow; and that the region recorded through both the green and violet filters is reproduced by the printing color of the red record, generallv a blue.
"Now, as one has, at present, to use reproduction colors which seldom give us what we require, we suggest that the amount which the red and green records fully overlap should be restricted to the region that corresponds most nearly in hue with the printing yellow to be used. Similarly, the completely overlapping region of the green and violet records should be confined to the part most nearly matching the blue or greenishblue printing color. We consider the overlapping should never exceed this, and, on the other hand, should never fall far short of it. The gradual overlaps required by filters, based upon color-sensation curves, or intended to follow color mixture curves, are seldom or never realized in practice. They are things which are talked about but seldom produced. Mr. Ives is the only person, as far as we know, who claims to have successfully used niters whose records correspond to color mixture curves. We must give deference to Mr. Ives' statements, as he is a practical as well as theoretical worker. But some photochromoscope filters of his which we have examined only give a very crude approximation to color mixture curves, and their excessive overlap is, we have found, fatal to their use for three-color printing.
"There is another point which seems to have been lost sight of altogether, and is, moreover, an objection which has not as much significance if the filters have fairly abrupt absorptions. In all molecular phenomena, i. e., in magnetism and electric response to stimulus to living or non-living bodies, in photographic action, etc., there is a period of ineffective stimulus, e. g., in the case of a photographic plate there is a certain minimum amount of light action below which no deposit of silver occurs. Suppose now that by a certain exposure to the spectrum with suitable filter and plate, one obtained a silver deposit whose density corresponded to, say, the red color mixture curve ; if we give any other exposure, then the shape of the curve ought to remain the same (except, perhaps, with great overexposure, when in any case all parts would tend to reach their maximum ljmit of density). But this would not be so, on reducing the exposure, the less dense parts would lose their density too rapidly and become clear glass while there was still a good deposit in the densest part."