The history of three-color photography (1925)

26 History of Three-Color Photography content is perceived, and they appear purer than they actually are. For instance, a green made by the admixture of equal parts of Paris blue and chrome yellow, contains, according to the color law, about three-fourths black, and if our sensation followed the law of color mixture, it would be a greenish grey. Since, however, the presence of 75 per cent black does not give the subjective luminosity of 0.25, but 0.64, the black sensation is so suppressed that the compound green still appears sufficiently pure. The Theoretical Printing Inks. — If we use the chromatic circle as the starting point of our search for the correct inks we can at once determine the hues, and there should be theoretically any number that one could use. But as one of the fundamental facts is that the colors must be so chosen that they are as far removed from black as possible, and further that they shall include the whole of the spectrum colors, it becomes obvious that they must lie on the periphery of our circle at 120 degrees apart. And as we can not make yellow by the admixture of pigments this gives us our initial starting point, the one fundamental color that is fixed for us. The color that should be chosen for this should be a pure "lemon-yellow chrome" pigment, without any greenish or reddish tinge. The blue at 120 degrees from this is known as peacock blue, and the corresponding red is a crimson, pink or magenta. F. E. Ives40 considered that to fulfill his theoretical requirements, the inks must be perfectly transparent, and as specifically anti-chromatic as possible to the respective primary colors in white light. The minus-red, or peacock blue ink, should have a strong absorption from the red end of the spectrum up to the D line and fall off to nothing in the greenishyellow. The minus-green or crimson, should show a strong and even absorption from over D to F, falling quickly to nothing outside these limits. The minus-blue or yellow ink should have a strong absorption in the violet and blue and fall off gradually between the F and E lines. C. G. Zander,41 who was a practical ink maker, stated that good lakes, that is dyes precipitated on an earthy base, such as aluminum hydrate, are the most suitable, as they distribute well, print even and do not clog up the interstices between the dots of the printing block. All the alizarin lakes, the so-called artificial madders, are perfectly permanent, even in tints, and can be produced in all shades from carmin to scarlet and purple. He defined the ideal inks, as regards color, as a "pure red pigment, one that is neither a purple nor an orange, that is it represents as nearly as possible the fundamental red sensation of the spectrum; a pure yellow, not inclined to orange or green, that is about the shade of sulphur, or as artists call it 'lemon-yellow' ; the third, a neutral blue, in which neither the violet nor the green predominate. This will be the shade of cobalt blue." One of the most important attributes of the inks is transparency, for if they are not transparent then the resultant color of a mixture will be that