106 Prof. Miller on the Forms of Sulphur et o/NicJcel, S^c, 



minimum tleviation of a ray polarized in the plane T C A is 

 about 42°. 



Asparagine. — Prismatic. The number of crystals at my 

 disposal was too small to ensure great accuracy in the deter- 

 mination of the angles. 

 B(0;1;0), P(l;l;l), M(1;1;0), N(2;l;0), K(1;0;1). 



B M = 39°47' 

 B N = 59 1 

 K K = 50 42 

 M P = 53 29 

 BP = 62 47 

 KP = 27 13 

 N P = 55 49 

 K N = 68 28 

 XP = 67 37 



The ratio of the velocity of light in air to its velocity within 

 tlie crystal is 



1*623, for a ray perpendicular to B B, polarized in a plane 



perpendicular to B B ; 

 1 '600, for a ray perpendicular to Z Z, polarized in a plane 



perpendicular to Z Z ; 

 1 '583, for a ray perpendicular to X X, polarized in a plane 



perpendicular to Z Z. 



Carhazotate of Potash, — Prismatic. 

 MM= 89°36' 

 AM = 55 12 

 LL = 40 20 

 A L = 60 50 



The crystals are elongated in the a 

 direction of the axis of the zone 

 A M, and the faces L are so small 

 that their position cannot be accu- 

 rately ascertained. The doubly re- 

 fractive energy of this salt is pro- 

 bably greater than that of any other crystal. When yellow 

 light is refracted through the faces M M, the minimum devia- 

 tion of a ray polarized in the plane MA M is 51° 40'. Hence 

 the ratio of the velocity of light in air to its velocity within 

 the crystal is 1*527, for a ray in the plane MAM polarized 

 in the same plane. For a ray traversing the crystal in the 

 direction Y Y, and polarized in a plane perpendicular to 

 MAM, this ratio is found to be nearly 1 -95. 



