2 Oliver H. Gish 



silvered portion of .the plate showed a shift relative to those from 

 the unsilvered part. From the relative shift the phase change was 

 determined. 



Wiener later by this method studied silver and obtained results 

 that were in marked disagreement with those of Wernicke. 



The work of these men was followed by a more exhaustive 

 study, both theoretical and experimental, by Drude. In his meas- 

 urements Drude used a wedge of glass so thin that it showed 

 interference bands when illuminated by monochromatic light. 

 The shift of the bands w'as then measured with a cathetometer. 

 His measurements were in close agreement with those of Wiener 

 and he suggested that the lack of agreement with Wernicke was 

 due to the difference in the silver films. 



It was known to Wernicke that the phase change varied with 

 the thickness of the silver film, and he made use of this in an 

 attempt to determine the direction of the shift of the interference 

 bands, but Wiener was the first to make an extended study of the 

 variation of the phase change with the thickness of the silver 

 film. He found that with increasing thickness, the phase change 

 increased until a full silver was obtained after which it remained 

 constant, with a value approximately that for massive silver. In 

 addition to the work on thick films Drude investigated films of 

 varying thickness and obtained results that were in close agree- 

 ment with those of Wiener. To explain this phenomenon he 

 assumed that over the reflecting surface of the silver was a very 

 thin film (about one four-hundredth of a wave-length in thick- 

 ness) having abnormal optical constants. Then when the silver 

 film is sufficiently thick the light reflected from the normal silver 

 back of this thin film is the more intense and consequently the 

 eft'ect of the latter is not observed. 



Koenigsberger and Bender repeated the work of Drude with 

 modifications designed to avoid surface films, and obtained results 

 for gold and platinum, supposedly free of films, that were similar 

 to those of Drude for silver with the assumed films. Hence they 

 questioned the validity of Drude's explanation, suggesting rather, 

 that instead of applying, as Drude did, a system of equations in 

 which multiple reflections are neglected, more general forms such 



1 68 



