EECENT PROGRESS IN OPTICS. 127 



obtained a rose-colored form of silver photochloride which "in the violet 

 of the spectrum assumed a pure violet color, in the blue it acquired 

 a slate blue, in green and yellow a bleaching" influence was shown, in 

 the red it remained unchanged." But in the absence of any means 

 of fixing these colors, a promising prospect brings disappointment. 



While it is abundantly possible that colored illumination upon suit- 

 able color-receptive materials can give rise to similar body colors, we 

 are still far from having these materials under control. There seems 

 at present to be greater promise in another and quite different applica- 

 tion of optical principles. The suggestion appears to have been first 

 named by Maxwell (Eoyal Institution lecture, May 17, 1861) in 1861 

 that photography in colors would be possible if sensitizing substances 

 were discovered, each sensitive to only a single primary color. 



Three negatives might be obtained, one in each color; and three 

 complementary positives from these, when superposed and carefully 

 adjusted, would present a combination that includes all the colors of 

 nature. In 1873 H. W. Yogel in Berlin discovered that silver bromide, 

 by treatment with certain aniline dyes, notably eosine and cyanine 

 blue, can be made sensitive to waves of much longer period than those 

 hitherto effective in photography. In 1885 he proposed to sensitize 

 plates for each of a number of successive regions in the spectrum, and 

 to make as many complementary pigment prints as negatives, which 

 should then be superimposed. This somewhat complicated plan proved 

 difficult in practice. In 1888 P. E. Ives (Journal of the Franklin Insti- 

 tute, January, 1889), of Philadelphia, adopting the more simple Helm- 

 holtz-Maxwell modification of Young's theory of color, applied it to the 

 preparation of suitable compound color screens which were carefully 

 adjusted to secure correspondence with Maxwell's intensity curves for 

 the primary colors. The result was a good reproduction of the solar 

 spectrum. But to reproduce the compound hues of nature it is neces- 

 sary specially to recognize the fact that although the spectrum is made 

 up of an infinite number of successive hues, the three color sensations 

 in the eye are most powerfully excited by combinations rather than by 

 simple spectral hues. Thus, according to Maxwell's curves, the sensa- 

 tion of red is excited more strongly by the orange rays than by the 

 brightest red rays, but the green sensation is excited at the same time. 

 This fact has to be applied in the preparation of the negatives, while 

 images or prints from these must be made with colors that represent 

 only the primary color sensations. Properly selected color screens 

 must, therefore, be used for transmission of light to plates sensitized 

 with suitable aniline dyes, and the adjustment of ratios with this end 

 in view is not easy; but it has been successfully accomplished. From 

 three negatives thus made, each in its proper tint, positives are secured ; 

 and these are projected, each through its appropriate color screen, to 

 the same area upon a white screen. The addition of lights thus sent 

 from the triple lantern gives the original tints with great fidelity. 



