192 



conclusion arrived at is therefore, that the optic axial plane 

 makes a right angle with a^o and an ideal transverse section 

 of a single crystal will accordingly appear as shown on the 

 figure : 



_Y 



X 



Fig. 2. Britholite. 



The pyramidal faces on the apparently hexagonal crystal are 

 partly brachydomes, partly real pyramidal faces. The larger 

 ones among them are almost always brachydomes. The pyra- 

 midal faces are rare and belong always to the face а^. 



The faces on the separate individuals, which were examined, 

 are the following : 



{010} {110} {130} {021} {111} 



and the following angles are measured: 









Calculated 



(010) 



(110) = 



*58° 13' 



— 



(010) 



(130) = 



28° U' 



28° 17' 



(021) 



(010) = 



*/j9°48' 



— 



(111) 



(110)= 



с 5F 



51° 16' 



From this the following calculation is made : 

 ä:b:c = 0-620: 1 : 0423. 



