25C 



BULLETIN OF THE UNIVERSITY OF WISCONSIN. 



Rowland's concave grating. Between the grating and its focus- 

 he placed the small cyanin biprism with its refracting edge par- 

 allel to the lines of the grating. The double set of bright lines 

 thus formed at the focus were photographed. The distance be- 

 tween the corresponding lines in the two sets were measured and 

 from these measurements the refractive indices were calculated. 

 These results are given in the following table : 



The objective point of Pfliiger's work was to test the Ket- 

 teler-Helmholtz dispersion formula, the derivation and form of 

 which will be given in the theoretical part of this paper. The 

 equations require experimental data for both the extinction and 

 refraction indices. To find the former, Pfluger employed a 

 Konig's spectro-photometer, by which the absorption of the light, 

 when passing through a thin film of cyanin, was measured. 

 The ratio between the original intensity, I, and the strength af- 

 ter passing through the film I', is found by equation (1), where 

 a is given by the reading of the photometer. 



I' — Cot a 



(D* 



The extinction index, X, is then calculated by the usual form- 

 ula-, (2) , where d is the thickness of the film, X the wave length 

 of the light in free space, and e the base of the natural system of 



logarithms. 



I 4ffXd 



T = e- 



A 



(2). 



In the series of papers published by Pfluger are found several 

 tables for the extinction index. The disagreement between the 

 calculated and observed values led him to repeat this work with 



