4 Jarl A. Wasastjerna. (LXIV 



X indicating the coefficient of absorption, and x the thickness 

 of the plate in mm. The meaning of Iq^, Iq^, I^ and I^ 

 becomes at once obvious. Further I indicate by Iqq the 

 intensity of that beam which passes the crystal and is used 

 by comparison to determine the absorption of the extra- 

 ordinary ray which goes through the crystal. 

 Now if we adopt the formulae 



-'oc ^^ ^1 ' -'oo ' I o 



-'oca ^^ ^2 ' ^0£ ' j 



where C^ and Cg are constants and where the former equation 

 appUes to the direct determination of x^, whilst the latter 

 equation refers to the relative determination (namely of 

 Xft,— Xg, as will be shown låter), we obtain 



-'£ — ^1 -'oo t^ 5 I o 



-'oo ~^ ^2 ^oe '^ » ) 



and finally the formulae: 



e^«-^ = Ci .cot2y, a\ 



e(>f(B-J^f).x_c'^ .cot^y. bj 



The constants Q and Cg will, according to (2) either signify 



C or ^ • That depends, of course, in the case of constant 



Cl, on whether Iqq > Iq^ or Iqq < Iq^ and in the case of 

 constant C^ on whether Iq^^ > /^g or I^^^ < Iq£ . For this 

 reason it is necessary in each experiment to take note of the 

 indicated differences. 



According to (4 a) we calculate jt^ and according to (4 b) 

 (x^ — Xj). The sum of these' expressions, that is to say 

 Xf^ , must be constant, whereby we can check the x^ values 

 obtained. 



