8 Oliver H. Gish 



the phase changes from glass in air and from fuchsin in air are the 

 same for the red, the relative shift of the bands is a direct measure 

 of the thickness of the film. Values obtained by this method from 

 adjacent parts of one film were 275 and 333 /-t/A. The films of 

 eosin and aniline-orange were so thin as to show Newton's colors. 



Observations 



Silver. — The photographs taken for silver on mica showed a 

 relative shift of about three tenths of a band at 580 /x/a, decreasing 

 slowly with decreasing wave-length to about 542 /x/a where the two 

 sets of bands seem to coincide. The shift with silver on glass was 

 practically the same as for mica except that the position at which 

 the bands coincide seemed to be shifted farther toward the ultra- 

 violet (to about 390/u,/x). They continued to coincide throughout 

 the transmission region for silver and as far as the photograph 

 showed distinct bands (296 /x/x). Because of the high reflection 

 coefficient of silver the silvered portion did not give well-defined 

 bands in the greater part of the spectrum. The accuracy with 

 which these could be measured did not justify more than qualita- 

 tive observations. 



Drude's formula^° for the absolute phase change (A) at the 

 boundary between a transparent medium of refractive index (Wj), 

 and an absorbing medium having an index of absorption {k) and 

 an idex of refraction (n') is: 



2nn-Ji 



n^ — n^k^ — n^ 



From this formula, values for the relative phase change were 

 calculated from values for n and k obtained by Minor. ^^ These 

 were in close agreement with the observed values for the red, but, 

 being practically constant down to about 320 /X|U,, they are in very 

 poor agreement with the observed values at the shorter wave- 

 lengths. In the region where the bands coincide the calculated 

 values ranged from .38 for 326 /x/x to .32 for 275 /x/x. Minor's 

 observations however were made on massive silver, mine on parti- 



10 P. Drude, Wied. Ann., 51, p. 87, 1894. 



11 Minor, Ann. d. Phys., 10, p. 617, 1903. 



