COLOE PHOTOGRAPHY. 191 



would remain unchanged for some color which it absorbs, and hence 

 can not reproduce this. If the ground colors are not monochromatic, 

 the monochromatic illumination which such an one reflects would either 

 be mainly incorrect or inaccurate in tone. Such an error is introduced, 

 also, if the products of decomposition are not regularly absorbing and 

 light-sensitive. Finally, if less tban three products of decomposition 

 result, or if their colors are not radically different, not all colors can be 

 reproduced. This remark has reference, also, to the white product of 

 decomposition. In its absence white can not be reproduced. 



In spite of all such deviations, any light-sensitive substance which 

 yields colored decomposition products will reproduce colors to a certain 

 extent; for the colored illumination will leave similarly colored com- 

 pounds unaltered, since they reflect the light, and will, on tbe other 

 hand, decompose other colored substances more readily, since they 

 absorb it. 



It will be thought that the properties of a color-receptive substance 

 are very complicated and difficult to attain. Nevertheless, this com- 

 plication is afforded by known natural processes. It is, however, not 

 necessary, for if it is sought to attain color photography by body colors 

 in the simplest way, it is possible to make a selection of the absorbed 

 light which produces decomposition. I will return to this in Chapter 

 XIV. 



XIII. — Explanation of the Color Reproduction in Seebeck's 



AND POITEVIN'S PROCESSES. 



The color reproduction is explained by the fact that the light-sensi- 

 tive substances used possess to a certain degree of approximation the 

 qualities of a light-receptive substance,* not completely, for the color 

 reproduction is not complete. 



The first deviation consists in that the light-sensitive substance is 

 not black, but in Seebeck's process dark violet to gray violet, in Poite- 

 vin's a dark gray violet to gray brown. Black can therefore not be 

 reproduced, and in its place occur the above-mentioned dark hues. 

 Nevertheless, these substances share with black the characteristic that 

 they absorb all visible rays to a certain measure and are light sensitive 

 to all. The products of decomposition are, as remarked in Chapter I, 

 substances of different colors. They must be, according to the results 

 of Carey Lea and Krone, either very numerous or very different in 

 color. But they are not absolutely monochromatic, and in this is to be 

 found the reason why the reproduction of color tones is partially incor- 

 rect. (See Chapter IV.) 



A white product of decomposition is not present in Seebeck's process, 

 hence white can not be thus reproduced. 



In Poitevin's process white can be reproduced. The tendency to the 

 production of white is, however, less than that for the other decompo- 

 sition products, and the colors are made pale only after long exposure. 



