270 SENARMONT ON THE OPTICAL CHARACTERS OF 



Measurement of the Angles included within the Axes, 



Measurement of the Indices, 



In the sulphates and the chromate the bisecting line is the 

 axis of maximum elasticity. 



Nitrate of Potash. 



KON^Qs. 



Arragonite. Carbonate of Lead. 



BO CQ\ 



The nitrate of potash crystallizes, according to Mr. Miller, in 

 right rhombic prisms of 118° 50', truncated by tangential faces 

 at the acute and obtuse edges, and bearing at the summits 

 several faces parallel to the minor diagonal of the base, together 

 with the faces of an octahedron with a rhombic base of 91° 46' 

 and 131° 22' resting upon the edges of that base. 



The plane of the optical axes is parallel to the major diagonal 

 of the base and to the height of the prism ; the bisecting line is 

 parallel to that height. 



Mr. Miller has likewise determined the three optical constants 

 of nitrate of potash and the angle included between the axes*. 

 He has found that the constants a, b, c bear an inverse propor- 

 ion to the numbers 1*333, 1*5046, and 1*5052; and that the 

 bisecting line is the axis of maximum elasticity. This result is 

 also verified by the usual methods of compensation. 



Arragonite crystallizes in right rhombic prisms of 116° 16', 

 truncated by tangential faces at the acute edges, and bearing at 

 their summits several faces parallel to the minor diagonal of the 

 rhombic base, together with the faces of an octahedron of 93° 30' 

 and 129° 37' resting upon the edges of that base. The plane of 

 * Poggendorflf's Annakfiy vol. 1. p. 376. 



