248 
Proceedings of the Royal Society 
tan «= 
(J-c)(6-cV3) 
putting 
B h 
Angular elements, — 
h 
a 
010, Oil = 42° 17' ; 001, 101 = 44° 25'. 
If these he altered thus, — 
010, Oil = 42° 30' (diff. of 13') ; 001, 101 = 44° (diff. of 25'), 
we have tan oj= -37134 
6j = 20° 30' nearly, which differs by ll'-5 from the ob- 
served value. 
15. Nitre . — Optic axes in plane c«, and make angles of about 3° 
with axis c. 
Angular elements, — 
100, 110 = 59° 25' ; 010, Oil = 40° 8'. 
If these be altered thus, — 
100, 110 = 59° 40' (diff. of 15') ; 010, 011 = 40° (diff. of 8'). 
The value of o) given by the formula. 
(6-c) (Z>-c,y/3) a 
is 3° 20' nearly, differing by about 20' from the observed angle. 
16. Cerussite . — In this mineral the optic axes lie in the plane he, 
and make angles of 4° 8' with axis c. 
Angular elements, — 
100, 110 = 58° 36'-5; 010, 011 = 40° 9'*5 ; 001, 101 = 35° 52'. 
-='6102 i = -723 f = l-1851. 
a a h 
Hence equation (3.) becomes 
, -3898 X -0569 C 
= •08 if 5 = 1. 
B 
<y = 4° 36' nearly, which differs by 28' from the observed value. 
If ^ = ^ =1‘1851, the equation becomes. 
tan ^ =-08xl-1851 = -0948. 
w = 5° 24', which differs from the observed value by 1° 16'. 
