OF OPTICAL ELASTICITY IN OBLIQUE-PRISMATIC CRYSTALS. 213 



(8). In Pyroxene, the faces being denoted by the same letters 

 as in the treatises of Mohs and Naumann, the symbols 

 of the simple forms are, p {00 1}, I {0 10}, r {100}, 

 m{110},/{8 10}, MI 01}, *{0 11}, u\21l\, 

 as {12 1}, o 1 2 1}, X {2 3 1}. 



The optic axes lie in a plane parallel to the 

 face /. The apparent direction of the optic axis a 

 seen in air through sections perpendicular to the faces 

 m, m', makes an angle of 74° with a normal to /. The apparent 

 direction of the optic axis /3 seen in water through the faces r, r, makes 

 an angle of 27°, 40', with a normal tor. M = 1.680. Hence ar' = 80°.34', 

 /3/= 21°. 38', £r'= 56°. 6'. 



The crystals of Pyroxene in which I first attempted to determine the 

 position of the optic axes were all twins composed of individuals of 

 unequal size, the twin-axis being perpendicular to the face r. Con- 

 sequently, a slice bounded by planes perpendicular to the faces m, m' 

 exhibited two systems of rings unequally bright, making with each 

 other an angle of 32°, which was bisected by the axis of the zone mr. 

 These rings were erroneously supposed to belong to the same crystal, 

 till the mistake was pointed out to me by Professor Norrenberg. The 

 best crystals which I have been able to procure for measurement give 

 pr = 74°. 20', nearly. In a twin crystal in Mr Brooke's collection, 

 the face t of one individual coincides accurately with the face p of 

 the other. This shews that Pyroxene may quantitatively be referred 

 to the right prismatic system. The position of the optic axes, as well 

 as the nature of the symmetry of the faces u, ss, o, X, prove, however, 

 clearly that it belongs to the oblique-prismatic system. 



(9). In crystals of Sugar the faces are too uneven to admit of de- 

 termining the angles they make with each other 

 nearer than within, perhaps, half a degree of the truth. 

 a being the face parallel to which a very distinct 

 cleavage exists, mm =79°.20', ar = ll6°.40', «c = 75 n .30', 

 nearly. The symbols of the simple forms are, a {10 0}, 

 c {0 1}, r{l 1}, * {1 1}, n {0 1 1}, p {1 1 1}. 



