246 
Proceedings of the Boycd Society 
Miller’s “ Mineralogy,” or elsewhere, the calculated agrees with 
the observed angle between the optic axes. 
We proceed to the comparison of these formulEe with observation. 
12. Ghrysoheryl. — In this mineral, the optic axes lie in the plane 
he, and make angles of 13° 55', with axis c. 
Let be this angle. 
c(i - 
Then tan <y = cot ui = - ^ ~ by formula (3.) 
Angular elements, — 
100, 110 = 64° 49'; 010,011 = 39°!'; 001, 101 = 30°?'. 
- = cot 64° 49' =-4702. 
a 
- = tan 30° 7'z=-5801- 
a 
tan oi = 
•5298 X -1856 G 
•4199 X 1 152 "'B 
C 
= •20326 if ^ = 1, 
B 
(«•) 
On this supposition, -w is somewhat less than 11° 30', which 
differs by 2° 25' from the observed value of 13° 55' ; this difference 
is too great to be ascribed either to errors in the determination of 
the angular elements, or of the angle between the optic axes. 
0 , ^ 
Hence ^ is not equal to unity. 
C 
The value of — must be such that the value of a given by equa- 
tion («) is equaJ, or nearly so, to 13° 55'. 
We find this to be the case on triah if _ = 
’ B h 
For chrysoberyl -| = cot 39° 1' = 1-2337. 
Hence, equation (a) becomes 
tan 61 = •20326 x 1-2337. 
= •2507. 
6i = 14°4'-5; 
