192 COLOR PHOTOGRAPHY. 



It remains now to see for each of these processes whether the decom- 

 position products are regularly absorbing light-sensitive substances. 

 That this is the case only in a measure is shown by the degree of 

 accuracy of the color reproduction. 



In Seebeck's process the red is the most distinct. In order that it 

 can be produced with red illumination all the other decomposition col- 

 ors must be red sensitive so that they may be decomposed by red. 

 This is the case. 



As a test the result of an exposure to illumination by the spectrum 

 was turned in its plane through 90° so that each color of the picture 

 was exposed to the illumination of the whole spectrum. The red of 

 the first exposure was the only color remaining unchanged under the 

 red of the second exposure. All the other colors were destroyed and 

 the plate took on a red coloring to the borders of the ultra violet. 



The other colors behaved similarly, but as they were not so well 

 marked as the red after the first exposure, so after the second they 

 were even less distinct. It may, however, be said that the red pro- 

 duced by the first exposure was destroyed by the green and blue of 

 the second, though the lightening up of the ground tone in connection 

 with the red still remained. This agrees with the experiment of Carey 

 Lea, mentioned on page 172. The green of the first was destroyed by 

 the blue as well as by the red of the second. The blue of the second 

 exposure destroyed, therefore, both the red and green resulting from 

 the first. Violet could naturally do the same, as blue is produced 

 from the violet ground color. Since now yellow is scarcely at all repro- 

 duced in this process, the formation of blue by the action of blue light 

 is explained from the fact that it is able to alter all other decomposi- 

 tion products. Blue is, indeed, after red, the most satisfactorily repro- 

 duced. 



In Poitevin's process the colors are better marked, and the experi- 

 ment with crossed spectra was therefore more satisfactorily performed. 



In one experiment the exposures were each continued a half hour. 

 The colors of the first picture remained, as was expected, unchanged 

 where the same colors fell upon them in the second exposure. Under 

 different colored illumination they were changed according to the 

 observation of Dr. Holzapfel in the following manner: The red in t\\e 

 first picture was in the yellow of the second illumination yellow, and 

 under the other kinds of illumination correspondingly altered. 



The yellow of the first picture was unchanged by the red of tire 

 second illumination, was changed a little by the green, was greenish 

 in the blue, and was in the violet destroyed. 



The green of the first picture was changed to red under the red of 

 the second illumination, was yellow under yellow, and remained un- 

 changed under blue and violet. 



The blue of the first picture was made red under the red of the 

 second illumination, yellow under yello\v, green under green, and was 

 in the violet altered and darker. 



