OPTICAL ELASTICITY IN OBLIQUE-PRISMATIC CRYSTALS. 435 



EXAMINATION OF VARIOUS CRYSTALS ACCORDING TO THE METHODS 



ABOVE EXPLAINED. 



(1). Sulphate of Oxide of Iron and Ammonia. According to Mit- 

 scherlich (Jahresbericht 13), the composition of this salt, which belongs 

 to an extensive plesiomorphous group, is expresssed by the formula 

 H'^NS ■\- FeS -^ 1 H. Fig. 2. represents the poles of its faces. Their 

 symbols are A{1; 0; 0), C(0; 0; 1), H{0; 1 ; 1 ;) M{1', 1; 0), 

 P(l; 1; 1), Q(-l; 1; 1), T{2; 0; 1). 



When yellow light is refracted through the faces TC in the plane 

 AC A', the minimum deviation of a ray polarized in the plane AC A', 

 is 41", 26'. The apparent direction of the optic axis aa in air, when 

 seen through the faces TT', makes an angle of 7°,10' with 2'2"; 

 and the optic axes appear to be inclined to each other at an angle of 

 79" when the crystal is immersed in oil, of which the index of re- 

 fraction is 1,47. From these data we find Ta = 4'',47', Tfi = 71°,2', 

 r^ = 33'',8', A^ = 9'',6'. 



Tan T^ is nearly equal to 4tan>4f. The value of A^ deduced from 

 the equations tan Tf = 4tan^^, T^ + ^^ = 42°, 14' is 9°, 13'i. This 

 would make Q = 82'',25'i. Now, 46" tan 9'^ 13'^ = tan 82'',22'| ; therefore, 

 if we refer the faces T, A, C, to the rectangular axes ff, YV, ^^', 

 neglecting the difference of 3' in the value of C^, their simplest symbols 

 will be (1; 0; 1), (4; 0; -1), (2 ; ; -23). The magnitude of the last 

 index renders the hypothesis that ^f , ^^' are crystallographic axes highly 

 improbable. 



(2). The composition of Tartrate of Ammo7iia is expressed, according 

 to Dulk, (Jahrbuch fiir Chemie und Physik, 1831. B. 1.) by the for- 

 mula H^NT+^H. The poles of its faces are represented in Fig. 3. 



Vol. V. Paet III. sL 



