486 PROFESSOR MILLER, ON THE POSITION OF THE AXES OF 



A{i; 0; 0), C(0; 0; 1), H{0; 1; 1), 2^(1; 0; 1), L{-1', 0; 1), 

 3f(l; 1; 0), P(l; 1; 1), Q(-l; 1 ; 1). 



Cleavage parallel to the face A. 



AK 520,31' AM 55»,2' Qd gr^a?' QA' 6o»,54'i 



KC 39,53 HH' 81, 4.6 QL 41 ,l6^ CP 55,34 



CL 38 CH 49 ,7 ^i* 63 ,22 PM 35 ,48^ 



L^' 49,36 PP' 94,55 PIT 28 ,12 JWQ' 34 ,53 



iIfil/'69,56 PX 42,32 /TQ 27,31i QC 53,44-i. 



Z> = 25'',17', the light being refracted through CK. The apparent 

 angle in air between the optic axes aa and AA', is 4°,55'. In oil, 

 the index of refraction of which is 1,741, the apparent angle between 

 the optic axes =42'',20'. This gives ^0 = 8", 7', A(i = 35'',54>', A^=W,M', 

 i'^ = 33°,12. 



In this case the positions of some of the faces A, K, C, L must be 

 altered half a degree before they can be referred to the rectangular 

 axes ff, W, ^^' with tolerably simple indices. 



(3). A solution of Benzoic acid in alcohol, when suffered to eva- 

 porate, affords crystals of which the faces C, K, I (Fig. 4) alone are 

 bright. Cifr= 69",25', C/=97'',20' nearly. Z) = 64°,45', refraction taking 

 place through the faces CK. The apparent direction of ad in air 

 when seen through CC makes with CC an angle of 4°, 30'. When 

 immersed in oil of which the index of refraction is 1,471, the appa- 

 rent angle between the optic axes is 75". Hence Ca =2'',47', C/3=59'',50', 

 C^=28»,31', ^^=40^54'. 



tan K^, tan I^, tan C^ are nearly as the numbers 3, 1, 5. 



The equation 1 tan /iT^ = tan 7^ = |^ tan C^ is satisfied by making Cf= 

 27°,56'^, JC = 97'',17'. Hence the faces C, I, K may be referred to the 

 rectangular axes ^f, YY , l^ without greatly altering the observed 

 angles, and their symbols will be (-1 ; 0; 5), (1 ; 0; 1), (1 ; ; 3) re- 

 spectively. 



