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46
History of Three-Color Photography
filters, it may be as well to record some theoretical data, which are of considerable practical importance.
Theory of Filters. — Von Hiibl,3 who has paid special attention to the manufacture of niters, gives some useful hints, which are summarized in the following notes.
The first point is the change of color with increasing quantity of dye. In Fig. 8 is shown the hypothetical case of a bright yellow dye, which we
K64 K32
5500
3500
may assume to be in separate sheets. With one sheet the absorption may be represented by Kl, and it will be seen to absorb the violet and blue in decreasing amount till at E it is quite transparent, the resultant color will be a pale or whitish yellow. But if we add another film or thickness, the absorption becomes K2, and the yellow appears more saturated or deeper, because more violet and blue, which paled the first hue, are absorbed. With four films, as in K4, the color deepens still more, as the blue and violet are reduced by the amount shown by the vertical NM, and so much so that the eye no longer recognizes their presence. This also applies to eight films, as in K8, but the change of color is still less apparent. And with the addition of further films, that is with sixteen to sixty-four, there is no apparent increase of color, though the luminosity may not be so great. Thus at first, with each doubling of the films there is marked color change, which becomes less and less with further increase, and the practical limit of the absorption lies at E.
With some dyes, particularly those with rather flat curves, there is, with increase of thickness of the dye film, an increase of the absorption, especially towards the red end, so that the color deepens somewhat and