260 M. G, Wertheim on the double Refraction 



method is more direct than the preceding one; but it often 

 leaves more uncertainty, because of the invariably incomplete 

 extinction of the image E. 



Suppose, in the first place, that two compressions had been 

 combined. 



Let P„ be the charge which it is necessary to apply to the 

 compensating piece placed on Aj, so that the double 



Pj and P, two equivalent weights, the first of which has been 

 removed from the press Aj, and the second placed upon the 

 press A^; 



Pj the weight sought, which produces in the piece placed on 



Ag the double refraction measured by ~ ; 



then we have . 



P 



P4= p-I*a. 



This formula applies equally to the traction ; only if we ope- 

 rate according to the second method, which is that of the com- 

 plete compensation of two double refractions. Pa represents the 



charge which produces, by pressure, the difference of path -^, 



These compensations also furnish us with a means of verifying 

 the exactitude of an hypothesis which we have implicitly ad- 

 mitted in the construction of our table. After having demon- 

 strated that the differences of path which correspond to all charges 

 may be found by means of intei'polations between the fixed 



points -^, ~, 2-^, 2-^ .... y we have afterwards made the 



same interpolations between and ^ ; but this latter we were 



2 



not authorized to do, and the accuracy of the procedure has not 

 thus far been verified, save by the coincidence of our determina- 

 tions with the measurements of Newton. 



We can now fill this gap ; for this purpose we place in the 

 two presses two pieces, for which the values P« and P/, have been 

 previously determined by direct experiment. We compress the 

 former up to the teinte sensible, and we determine afterwards the 

 equivalent charges P, and Pg which correspond to the different 



double refractions comprised between and -^ ; if between these 



two limits the optic compressibility is subject to any notable 



