OF OPTICAL ELASTICITY IN OBLIQUE-PRISMATIC CRYSTALS. 211 



about 18°. 40', with a normal to the face x. n = 1.631. Hence a/3 = 30°. 22', 

 and the axis of elasticity £ coincides with the axis of the zone xfq. 



(3). In Phosphate of Soda, Na*PH*, according to Mitscherlich 

 (Annates de Chimie, Tome 19.) ur — 33°. 8', rp = 

 25°. 24', pf = 50°. 48', fb = 33°. 25', bu = 37°. 17', 

 dm = 33°. 55', di = 65". 4', dn = 53°. l£, dl = 52°. 9', 

 dt = 36°. 30', dk = 67°. 6', pt = 6T. 55', pn = 31°. 30', 

 pm — 73°. 3'. The symbols of the simple forms 

 are, k {10 0}, d {0 1 0}, p \0 1\, m \1 1 0}, 

 n \1 11}, t {111}, A {201},/ {101}, /• {10 1}, 

 Z {0 2 3}, k {3 1 3}, i {3 1 0}. 



The optic axes lie in a plane perpendicular to the faces u, p, f. When 

 the crystal is immersed in oil, the apparent direction of the optic axis a seen 

 through the faces p makes an angle of 34°. 30' with a normal to p, and an 

 angle of 58°. 40' with the apparent direction of the optic axis /3 seen through 

 artificial surfaces nearly perpendicular to the optic axis (Z. n = 1.40 nearly. 

 Hence jo a = 36°. 30', jo/3 = 93°. 10', /j£ = 64°.50'. Therefore the axis of 

 elasticity f very nearly coincides with the axis of the zone rnd. It is not 

 possible to determine the positions of the optic axes of phosphate of soda 

 very accurately on account of its feeble double refraction, the imperfection 

 of its surfaces, and its tendency to effloresce. 



(4). In Acetate of Soda, NaAH 6 , ap = 76".25', ph = 68".l&, ha = 

 35°. 15', mk = 42°. 15', pm = 75°. 35', pf = 420.43, pe = 60\ 22', pg = 

 81°.8',5. The symbols of the simple forms are, h {1 0}, i {0 10}, 

 p\001\, a {201}, / {111}, e {111}, g {2 2 1}. 



When the crystal is immersed in oil, the apparent 

 directions of the optic axes seen through a slice 

 bounded by artificial surfaces nearly parallel to the 

 faces a, make with each other an angle of 62°. 30'; 

 and the apparent direction of the optic axis /3 seen 

 through artificial sections nearly perpendicular to fi, 

 makes an angle of 80°. 30', with a normal to p. 

 m = 1.464. Hence a/3 = 117°.10', p% = 11°.9', «£ = 2°. 26'. 



DD 2 



