10 



IV. B. Cartmel 



at the violet end of the spectrum, and as (fi — i) is the quantity 

 that is measured, it does not matter very much whether the cor- 

 rection is made or not, considering that there are much graver 

 errors which are unavoidable. 



Knowing n\, the retardation produced by the fuchsin in light 

 of any wave-length, we may determine /*, the index of refraction 

 of fuchsin for light of that wave length, by the formula 



provided we know t, the thickness of the fuchsin film. This was 

 determined by a method which will be described later. The mea- 

 surements were made upon a film whose thickness was 580 f^i, 

 with the exception of those values falling between A=46o fifi and 

 A=5go/x,/x, which were made upon a film "192 )tx/x thick. 



The results are plotted in plate I, and in the following table 

 they are compared with the values obtained by Pfliiger^ by the 

 prism method, and by Walter ^ by the method of total reflection: 



The curve is uncertain within the absorption band and perhaps 

 also to some extent in the violet, but in the red end of the spec- 



'^Loc. cit. 



"^Wied. Ann., vol. 57, p. 396. 1896. 



no 



