214 



PROFESSOR MILLER ON THE POSITION OF THE AXES 



The apparent directions of the optic axes a, /3, seen in oil through 

 the faces a, a, make angles of 1°.32' and 49°.58', with a normal to a. 

 iu = 1.57. Hence a a = 1°.26\ fl/3 = 45°.50', a% = 22°. 12', r^ = 4°.28'. 



(10). In Tartrate of Potash, KTH-, the cleavages being parallel to 

 m, t, me = 37°. 47', et = 52°.42', Z>6' = 45°.20'. The symbols of the 

 simple forms are, c \0 1 0}, e |0 l\, t \\ 1$, 



IB {10 1}, * \0 1 1}. 



The apparent directions of the optic axes seen in oil 

 through the faces t lie in a plane perpendicular to the 

 face c, making an angle of 67°. 30' with the face /. 

 They make with each other an angle of 64°. 45', and 

 therefore angles of 38°. 43', with a normal to t. n = 1.526 

 nearly. Hence, supposing a ray in the direction of the optic axes to 

 be refracted in the same manner as at the surface of glass, having 1.526 

 for its index of refraction, t% m 21°. 20', a/3 = 118° nearly. 



The above assumption, though not strictly correct, will not occasion 

 any considerable error in the present instance. This appears to be the only 

 practicable method of determining (approximately) the positions of the 

 optic axes, when the plane in which they lie is not perpendicular to 

 the faces through which they are seen. It is used in the two following- 

 cases. 



(11). In Chlorate of Potash, KCl, the cleavages being parallel to 

 the faces m, m', mm' = 104°. 0', ee' = 79°.30', pm = 74°.30'. The sym- 

 bols of the simple forms are, p J0 1J, m \1 10}, 

 e {0 1 1}, c {I 1}. 



The apparent directions of the optic axes seen in oil 

 through the faces p lie in a plane parallel to the axis of 

 the zone pc, making an angle of 52° with the face p, 

 and they make with each other an angle of 28°. 15'. 

 m = 1.507 nearly. Hence pi = 37°.42', a/3 - 152°.30' 

 nearly. 



