PHOTOGRAPHY IN THE COLORS OP NATURE. 161 



from the same kind of negatives could be made to furnish such a per- 

 fect solution. A year ago I also believed that there were theoreti(;al 

 difficulties in the way of realizing a i^erfect process with color prints. 

 < )nly recently have I succeeded in showing what relation the colors of 

 the prints must bear to the colors of light used in projection, in order 

 to perform exactly the same function and, under like conditions of illu- 

 mination, secure ecjually perfect fulfillment of theoretical requirements. 



In the projecting method, we build up the luminous image by adding 

 light to light. White light is produced by the mixture of the three 

 colored lights used for projection, and black by their suppression 

 But when we carry out the process to produce permanent pictures, the 

 paper which may form the basis of the picture is itself white, and it is 

 the shadows that are built up by the super-position of color prints. 



Nevertheless, the color print has exactly the same function to perform 

 as the lantern positive, i. e., to absorb and suppress by its shading, 

 light affecting one primary color sensation. If we remove our tliree 

 positives from the lantern, the screen is evenly illuminated with white 

 light. If we then replace the one representing the green sensation, its 

 shadows will absorb the green light with the result that the screen bears 

 a picture in the complementary color, pink, on a white ground. In the 

 color-print method we commence with a white surface which corresponds 

 to the fully illnniinated screen, and the shadows of the color print repre- 

 senting the green sensation, when laid upon this surface, absorb the 

 same kind of rays as the shadows of the positive in the lantern, and 

 with the same result, a pink monochrome picture on a white ground. 

 Superposing tlie other two color prints upon the first one on paper is 

 like inserting the other two positives in the lantern. This explains why 

 the primary sensations are represented by ijrints having shades of the 

 comjjlementary (absorbing) color. It is the lights and not the shades 

 of the color prints that represent the effect upon the respective primary 

 color sensation. It is only necessary to use dyes that comjiletely absorb 

 red light but neither green nor blue-violet for the print representing 

 the red sensation, green but neither red nor blue-violet for the green 

 sensation, blue-violet but neither red nor green for the blue sensation, in 

 order to obtain from my negatives a color print heliochrome that exactly 

 fultills all theoretical requirements, provided that it be examined in the 

 same kind of white light that we obtain in the screen projections by 

 mixing red, green, and blue- violet rays. The dyes mentioned by me in 

 my ])aper of November 21, 1888 (Prussian blue, aniline-magenta, and 

 aniline-yellow), fulfill this requirement, and color-print heliochromes 

 made therewith according to my instructions must therefore re-produce 

 all the colors of nature under the conditions of illumination just stated. 



In order to obtain colors that would appear of exactly the right kind 

 and shade in ordinary white light, it would be necessary to use dyes 

 each of which completely absorbed all light affecting the color sensa- 

 tion ^^■hi<•h it represented, but no other. The colors would then be 

 correct in ordinary white light, but would appear too dark, relatively, 

 SM 93 11 



