182 COLOR PHOTOGRAPHY. 



VII. — Prism Experiment with Color Beproductions After 

 Becquerel — First Proof of their Interference Nature. 



Although the results of the previous chapter do not require experi- 

 mental verification, yet I may say that in a prism experiment with a 

 photograph of the spectrum by Lippmann's interference method I 

 observed a very considerable color change. lu a place where for nor- 

 mally reflected light about the color of the yellow sodium liues 

 appeared, the color when viewed through the prism appeared on the 

 border between blue and greenish blue, or about at the place of the 

 hydrogen liue H/3 (F). The angle of incidence was less than 45° and 

 the prism used had a refractive index n D = 1.52. 



In the following observations I used exclusively the already men- 

 tioned prism for which n D = 1.75, and at an angle of incidence of 45°. 



When one observed a line drawn in the middle of the yellow on a 

 Becquerel plate, the ground in its vicinity when viewed through the 

 prism appeared green. Another line drawn along the border between 

 green and blue appeared through the prism lying in the middle of the 

 blue. In another plate the mark was drawn on the boundary between 

 yellow and green and it formed the boundary between green and blue 

 as seen through the prism. 



The experiment was repeated with homogeneous illumination from a 

 sodium flame. There was then perceptible in the yellow of the photo- 

 graphic spectrum a bright strip of about 1.5 millimeters breadth, the 

 center of which appeared to be shifted about 2.1 millimeters toward 

 the red when the reference mark was undeviated. This value is the 

 mean of the measurements of several observers. The magnitude of 

 the displacement happened to be exactly the distance from the D to 

 the C line in the spectrum. From this follows : 



A D 589 _ 

 f* = f = 656 = °- 90 ' 



From this change in the wave length of the reflected light the index 

 of refraction of the photographic film may be obtained by substitution 

 in equation (3) with the value of % = 1.75. Such a computation gives 

 n 2 = 2.4. In a second plate a displacement of 1.2 millimeters was 

 observed, from which it followed that/ p i = 0.94 and n 2 = 3.1. 



That the index of refraction of the film should vary when they are 

 not prepared under exactly the same conditions is obvious, for the 

 value of the reflective index depends on the proportion of silver proto- 

 chloride to the silver chloride in the film. According to Chapter III the 

 latter is, however, probably the chief constituent of the film, so that it 

 would be unlikely that the index of refraction should greatly exceed 

 that of chloride of silver. This is by the observations of Wernicke, 1 

 n Sl — 2.06. It is therefore improbable that the index of refraction 



1 Annalen der Physik unci Chemie, 142: page 571, 1871. 



