COLOR PHOTOGRAPHY. 177 



Iii neither of the other processes are the colors so accurately repro- 

 duced, and they look dull. The Seebeck plates show besides violet 

 only blue and red distinctly, and the latter is a sort of rose red, the 

 former being often rather grayish. Green is very indistinct and yellow, 

 hardly to be distinguished. But there appears in their places a con- 

 siderable brightening of the violet background. The Poitevin process 

 is superior to that of Seebeck. All the colors appear, but there is a 

 predominating yellowish brown tone. The yellow parts of the spectrum 

 are, as reproduced, more of an orange color, similar to the color of a 

 paper soaked with potassium bichromate solution. 



V. — Incorrectness of the Explanation of the Reproduction 

 of Colors according to Schultz-Sellack. 



A transparent film of silver iodide produced by the action of iodine 

 on a silver mirror chemically precipitated on glass is, according to 

 Schultz Sellack, 1 not chemically changed by the action of light, since 

 there is nothing at hand to absorb the iodine. The surface is, on the 

 other hand, mechanically disintegrated to a very fine powder. 



[ observed such a film under the microscope, and determined the 

 diameter of the grains to be about 1 jx (thousandth of a millimeter) 

 with a space between them of from to 3 //. The yellow, unchanged 

 iodide of silver film was visible through them. 



The series of transmitted colors succeeding under the influence of 

 sunlight is, according to Schultz-Sellack, yellowish brown, dark brown 

 with stronger illumination, red, green, blue, bright bluish white ; finally, 

 the film is, with slight exposure, almost completely colorless and trans- 

 parent. I observed also such colors, but since a strong and steady 

 beam of sunlight was inaccessible to me, I used an electric light, with 

 which it was impossible to secure uniform action. Parts of the iodide 

 of silver film exposed equally long to light attained different colors. 



But only violet and ultraviolet rays are able to cause this mechanical 

 disintegration as I found in confirmation of the results of Schultz- 

 Sellack. Herein lies the possibility of forming different colors by 

 different intensities and durations of exposure. A representation in 

 different colors can therefore apparently be obtained "which is able 

 to distinguish the different intensities of violet light, which are trans- 

 mitted by red, green, and blue glass." 



These colors are held by Schultz-Sellack to be diffraction colors, 

 because they aj>pear most strongly when one observes the plates in a 

 darkened chamber opposite to a small light opening. Indeed, the 

 colors are very dull when viewed by diffuse light. But they fail to 

 show the characteristic property of diffraction. They do not appear in 

 a direction at an angle with the incident ray as with a grating, but 

 in the direction of the rays passing through and reflected. 



1 Sclmltz-Sellack. "On the chemical and mechanical change of the silver haloid 

 salts under the action of light." Annalen der Physik und Chemie, 143; page 439? 

 1871. 



SM 96 12 



