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Historical and Theoretical Data 23
tion of the red filter should be from 5850. The accepted transmission is from 7000 to 5900.
The Printing Inks. — Although much information has already been given as to the theoretical printing inks in the last section, it has been thought advisable to devote more specific attention to this subject; for the mere statement that the inks must be red, yellow and blue opens up an enormously wide field of conjecture. And there are other factors to be taken into consideration, especially if we have to deal with typographic reproductions, which are independent of the hue of the actual printing inks.
The Superposition Failure. — Von Hubl37 pointed out that it is not immaterial as regards the final effect whether two transparent colors are mixed together, or whether they are superposed, white paper being assumed to be the support. If the colors are absolutely transparent, and in not too thick films, then the same results are obtained ; but if these requirements are not perfectly fulfilled then the results are not in accord. As a matter of fact, when two films, either of stained gelatin or printing inks, are superposed, the upper color always predominates. For instance, if a sheet of paper is printed first with a yellow and then with a red dye, there should be formed possibly a yellowish-orange ; whereas actually a reddishorange is seen. To illustrate this, a sheet of paper was printed with chrome yellow, and then with rose Bengal lake, which is very transparent, so that part of the paper was covered only by the yellow, another part only by the pink and the two colors were superimposed in the middle. From this were cut three color-top sectors, one of the superposed colors, and the other two from the pure yellow and pure red. In order to obtain color equilibrium on the top between the compound color and its constituents, it was found necessary to add to the former 30 per cent of white. The compound color was thus the same as that obtained by mixing one part of the chrome yellow with two parts of rose Bengal, instead of equal parts as theory demanded.
The transparency of a film of color depends obviously on its density, and on this and on the saturation of the color depends the amount of the superposition failure. With a saturated red film the incident light-rays will be reflected before they reach the underlying yellow film ; whilst with a less saturated red film the underlying yellow color approaches almost its complete value. Thus it will be seen that irregularities of color may be caused, and if a scale of yellow is overprinted with an equal gradation of red, the lower tones will appear of a reddish-orange. Obviously this defect will appear more strongly with three superposed films, so that the lowest film of all will hardly have any visual effect in the deeper tones. This incompleteness of the admixture of superimposed layers of colors is one of the greatest difficulties in three-color printing; and is the reason why we so often have the impression that the three colors do not sufficiently