PHOTOGRAPHY IN THE COLORS OF NATUEE. 157 



ron's original plan. He was the first to make the color prints by the 

 collotype process, which led to the use of the term " chromo-collotype." 



In 1870, Cros* abandoned the idea that red, yellow, and blue are pri- 

 mary spectrum colors, but still held that there are three primary colors 

 and mixtures thereof, and that these primary colors are orauge, green, 

 and violet. Ijike Dnhauron, he decided to make negatives by light of 

 these colors and i)rints in blue, red, and yellow. 



In 1884, Dr. F. Stolze, of Berlin, made a series of investigations and 

 tried to solve the problem by devising a procedure more in accordance 

 with Young's theory of color.t He said: "Although the colors corre- 

 spond with certain external processes in nature, there is also no doubt 

 that color as such is nothing objective, but a subjective sensation, based 

 upon the peculiar irritation of the visual nerves by those external pro- 

 ceedings. We can, therefore, only hope to produce a picture in natural 

 colors when we are enabled to reproduce u])ou the same the proceedings 

 which furnish to us the color impression. The general idea of all colcu'S 

 being based upon the three principal colors, red, yellow, and blue, is 

 an erroneous one. Thomas Young - - - assunu'S tliat there are 

 three kinds of nerve fibers sensible to red, green, and violet. Objective 

 homogenous light excites all three; but with red the iirst is excited 

 strongly, the second and third weakly; with blue, the second and third 

 moderately strong, the first weakly; with violet, finally, the third 

 strongly, and the first and second weakly. If all three kinds of nerve 

 fibers are eijually strongly excited the impression of white light will 

 take place." 



This theory in accordance with which Dr. Stolze tried to devise a 

 theoretical solution of the problem is only partly correct, measurements 

 by Clerk Maxwell and others having shown that the red sensation is 

 neither afi'ected by blue-green, blue, or violet rays, nor the blue (vio- 

 let) sensation by red, orange, or yellow rays, nor the green sensation 

 by red or violet rays. Neither is it the red rays that chiefiy excite the 

 red sensation, nor the violet rays that chiefly excite the blue (violet) 

 sensation. 



As a result of elaborate calculations, which, it must be said, could 

 just as well have been made without any reference to Y^oung's theory 

 of color, Dr. Stolze came to the conclusion that if three suitable selec- 

 tive color screens Avere used in connection with color sensitive plates 

 three negatives of the spectrum might be obtained, from which i^rints 

 in cyan blue, carmine, and yellow, if superposed, would reproduce the 

 color eflect of the spectrum. He did not show how to make selective 

 color screens calculated to secure the right kind of negatives to carry 

 out this idea, iior state what should be the form of the intensity curves 

 in such negatives of the spectrum. He merely gave a table, showing 



* Bulletin of the French Photographic Society, 1879, p. 23. 

 t Anthonifo Plwloyraphic Bulletin, 1886, pp. 516, 555, 588, 647, G78, 



