282 



SENARMONT ON THE OPTICAL CHARACTERS OF 



2<l> the reciprocal inclination of the bisecting lines belonging 

 to each crystal; 0=90° -l-<^. 



Second Method, — The artificial plate parallel to the horizontal 

 diagonal of the rhombic base being always cut in such a manner 

 that its normal, near to the bisecting line, falls within the obtuse 

 angle a, if the figure (fig. 9) represents an ideal section of the 

 crystal perpendicularly to the same diagonal of the base ; 



X the angle included between the normal of the plate and the 

 normal to the face P ; 



I, i the angles which the optical axes make respectively with 

 the normal of the plate, the first directed in such a manner that 

 it deviates from the normal of P, the second in such a manner 

 as to approach it ; 



R, r the corresponding interior angles ; 



2(0 the interior angle of the optical axes ; 



/8 the angle of their bisecting line with the normal to the plate ; 



m the index of refraction of a ray polarized and situated in 

 the plane of the optical axes ; 



sin R = — sin I, 



m 



/9=ft) 



R 



— r 



2~' 



sinr= — sm «, 

 m 



0=:\-fi=:X- 



Determination of the Indices m. 



