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Historical and Theoretical Data 11
will not come into play. The production of grey is readily seen from the fact that a drawing executed in neutral grey, must under all circumstances, if suitable sensitizers and filters are used, give negatives of equal value ; grey places in the colored original will thus be equally dense in all three negatives, and will, therefore, print in all the colors of equal intensity. In order to prove whether the inks will give greys, it is only necessary to print from one negative in all three colors, and then with correctly chosen inks, greys will be the result. It is not easy to fulfill this requirement; if Paris blue, chrome yellow and eosin lake are chosen, a yellowish brown instead of a grey will be formed, because between the absorption bands of the blue and red inks there is a broad gap. Still less satisfactory results are obtained with a bluish eosin lake or alizarin, though none of the present inks is quite satisfactory.
As regards the absorptions of the inks it may be said that each should absorb about one-third of the spectrum and reflect two-thirds. These requirements are met with in a yellow that is neither reddish nor greenish, as in a mean chrome yellow ; in a Milori blue or lake of similar shade. A satisfactory red ink is actually wanting, and one must be content with a bluish eosin or alizarin lake.
The plates must be sensitized, in accordance with Vogel's law, for those regions which the printing inks absorb, therefore, the above statement practically determines the sensitizing regions. The plates must not thus be sensitized for one particular region only ; but each of them should be sensitive for about two-thirds of the spectrum. The ordinary undyed silver bromide plate does not correspond to this requirement completely, since its sensitiveness for the bright blue is too little. The sensitiveness for the red-printing plate should extend over the yellow-green, green and blue-green. The action of all eosins extends, as is well known, chiefly over the yellow-green and, therefore, such sensitized plates give the bluegreens and blues with too little density. A combination of sensitizers, or one that has not the familiar gap in the blue-green must be used. For the blue-print plate there must be sensitiveness for the orange and red, which can be fairly satisfactorily fulfilled.
Ives' Theory. — F. E. Ives20 advanced the theory that correct color photographs could only be obtained by utilizing the Maxwell curves as a foundation, and he stated : "My own method is perfectly distinct from Hauron's, in that I do not expose sensitive plates through "orange, green and violet glasses," and from Vogel's, in that I do not make a separate negative for each region of the spectrum but only three, and in such manner as to secure curves of intensity which correspond to the action of light rays upon the sets of nerve fibrils which produce color sensation. This, in fact, is my principle, which is undoubtedly new and true, and is carried out by exposing color-sensitive plates through compound color screens, which have been adjusted by experiment in photographing the