COLOR PHOTOGRAPHY. 179 



are from the beginning pure, though weak. The beginning of the 

 action, therefore, causes not the same but different colors. 



For a more convenient test I caused to be formed on a sensitive film 

 a series of spectra with corresponding colors opposite, but giving dif- 

 ferent exposures to the several impressions. For this purpose a shutter 

 was placed across the slit and opened upward. Thus, after the close 

 of the experiment, one could at a glance observe the action of the dif- 

 ferently colored illumination. It was shown conclusively in this way 

 that the several colors did not at the beginning of the exposure give 

 rise to one but to different colors, which were similar to the colors of 

 the illumination. 



The following are the observations of Professor Grotrian, with a 

 plate prepared by Becquerel's process : 



Field 1. One minute time of exposure. Trace of red. Yellow and green only noticed 

 when field 2 is also under observation. Blue absent. 



2. Two minutes. Red stronger; yellow and green to be recognized. Blue 



absent. 



3. Four minutes. Red, yellow, and green stronger. Bine scarcely to l»o 



recognized. 



4. Eight minutes. Red, yellow, and green stronger. Bine to be recognized. 



5. Sixteen minutes. Red, yellow, and green stronger. Blue still weak. 



6. Thirty-two minutes. All colors stronger. 



The action in the ultraviolet was first apparent in the second field; hence, after the 

 red. 



Red is here, the first distinctly visible color, and appears under the 

 influence of red light. This must, therefore, according to Schultz-Sal- 

 lack, be the most powerful acting color of all, and at the same time red 

 must be the first stage of the disintegration colors. Other colors, or 

 lighter stages in the disintegration coloring, can only be caused at first 

 by red light, and red coloring must be the first result of other colors of 

 illumination. 



Observation shows the reverse. The other colors are caused by other 

 colored illumination, while the red is not changed by continued red illu- 

 mination, but on the contrary grows stronger. 



The same experiment was performed with Seebeck's and Poitevin's 

 plates with the same result. Before the first color to appear had changed 

 the other colors appeared in their places. 



Schultz-Sellack's explanation of the formation of colors in the older 

 processes of color-photography by disintegration colors is, therefore, 

 incorrect. 



I do not in this, however, assert that the intensity of illumination is 

 without influence on the color produced. Such an influence is as appar- 

 ent as in the Lippmann process. 1 To disprove his assertion, it is not, 

 however, necessary to prove a complete independence of the colors from 

 the intensity of the light, but only to show the error in the relation 

 between them which his hypothesis requires. 



See for example Krone, Annalen der Physik und Chemie, 46, p. 428, 1892. 



