DOUBLE REFRACTION IN ISOMORPHOUS SUBSTANCES. 273 



This prism presents various modifications, among which are 

 distinguished those which replace the base by a bevelment of 

 105° 8', the edge of which is parallel to the major diagonal of 

 that base {a^ in sulphate of strontia, Levy and Dufrenoy). 



The plane of the optical axes is parallel to the height of the 

 prism and to the diagonal of its base, the bisecting line parallel 

 to this major diagonal. 



The values of the three optical constants a, b, c are, according 

 to Mr. Miller*, inversely proportional to the numbers 1*571, 

 1*576 and 1*614; the interior angle of the optical axis is 40° 37', 

 the bisecting line is therefore the axis of maximum elasticity. 



Sulphate of potash. Chromate of potash. 



BO, RO^. 



These two substances were both crystallized in longitudinally 

 striated prisms, in which the angle 73° 30' could readily be 

 recognized; the ends present smooth reflecting faces of an 

 octahedron of about 92° 30' and 112° 40' with truncations tan- 

 gential to the edges of that octahedron, which in sulphate of 

 potash, according to Mitscherlich, form an angle of 120° 24', 

 and in chromate of potash of 120° 41'. 



The plane of the optical axis is parallel to the rhombic base, and 

 the bisecting line parallel to the major diagonal of that base ; if, 

 therefore, the faces of the prisms of 120° 24' and 120° 41', which 

 truncate the edges of the octahedron, are made vertical, the plane 

 of the optical axes will be parallel to the height of the prism 

 and to the major diagonal of the new base, the bisecting line 

 parallel to that major diagonal. 



The rings may easily be observed through an artificial face 

 tangential to the edge of 73° 30', and the apparent angle of the 

 optical axes is thus found to be equal for sulphate of potash, to 

 100° 52', for the chromate to 92° 20'. 



The dispersion of the optical axes corresponding to different 

 colours, which is not very marked in sulphate of potash, is tole- 

 rably strong in the chromate, and the isochromatic curves are 

 orange outside and greenish inside. 



I have measured the indices of refraction of a ray polarized 

 in the direction of the plane of the optical axes. 

 * PoggendorfF's Annalen, vol. Iv. p. 525. 



