170 COLOR PHOTOGRAPHY. 



The index of refraction for pure chloride of silver is about 2, and for 

 compounds poorer in chlorine probably even greater. When therefore 

 a ray of light falls upon silver chloride even with a considerable angle 

 of incidence, it would pass within nearly perpendicular to the surface, 

 on account of the great index of refraction, and the difference in path 

 for interference as compared with direct incidence is only slightly 

 changed. The index of refraction of the sensitive layer in Lippmann's 

 process on the other hand, which consists chiefly of collodion or gela- 

 tine, is only about 1.5. Here the angle of incidence becomes of impor- 

 tance, and the colors change with it in a way not to be desired. 



The interference nature of the colors and the stratification of the 

 Lippman gelatine plates may be recognized in another way. Different 

 observers : breathed on such plates and saw appear in place of the 

 original colors others of greater wave-lengths. This showed that the 

 colors depend on a changeable distance, namely, that between the ele- 

 mentary mirrors within, which was increased by the swelling of the 

 gelatin. 2 



I have repeated this experiment, and before large audiences have 

 replaced the breathing on the plates by the use of a stream of vapor. 

 In the photograph of the spectrum thrown upon the wall the colors 

 were transformed with great rapidity in the direction of the violet end 

 of the spectrum- and returned again as the moisture was driven from 

 the gelatin film by a Bunsen burner. This experiment can not, how- 

 ever, be performed with the older processes. 



Zenker 3 sought to give the light rays in chloride of silver a greater 

 angle with the normal to the surface by sending them first through a 

 liquid of high index of refraction, but without result. This expedient 

 must fail so long as a plane parallel layer of such a substance is used. 

 For by the refraction in this the original angle of incidence is decreased. 

 Such a diminution can not occur when the beam of light enters the 

 bounding surface of the auxiliary substance at right angles. Thus a 

 new experiment was suggested. 



I used a right angled glass prism with an index of refraction of 1.75 

 for the D lines. This was laid with its hypotenuse surface upon the 

 color picture and the intervening air space filled up with a layer of ben- 

 zine. For light rays entering normal to the side surfaces an angle of 

 incidence of 45° is thus secured in the strongly refracting medium, and 

 the ray entering the chloride of silver must, therefore, form a consider- 



^eslin, Ann. dechim. et dephys. (6), 27, p. 381, 1892; Krone, "Darstellingdernatiir- 

 lichen Farben," p. 66; Valenta, "Die Photographie in natiirlichen Farben," p. 68; 

 Halle a. S./publisbed by Wilh. Knapp, 1894. 



2 Dr. Neuhau8s (Photogr. Rundschau, p. 295, 1895) mistakenly believed that be bad 

 found an objection to the applicability of Zenker's theory to Lippmann's process in 

 tbe observed magnitude of tbe grains in the undeveloped plates, which was found 

 to be 0.0003 millimeter. Not the absence of grains, as he supposes, but complete 

 transparency is requisite for the production of fixed light waves. 



"Zenker. Photochromie, p. 85. 



