143 



The vibration -direction falling in tlie obtuse angle ß is 

 the optic normal , for the optic axial plane lies almost parallel 

 to the third pinacoid. The first mean line (the acute bisectrix) 

 coincides with the crystallographic r/.-axis; and as this is at the 

 same time the direction of the greatest velocity of light, the 

 mineral is optically negative. 



For the determination of the optic constants a natural and 

 a prepared prism were used. The former was bounded by the 

 two faces (130) and (l30) and bisected by the plane of the 

 greatest and mean velocity of light. The refracting edge of 

 this prism coincides with the crystallographic c-axis , but not 

 exactly with the direction of the mean velocity of light of the 

 substance. The deviation, which is 3^^23^ as above slated, gives 

 rise to an error in the determination. It cannot, however, 

 exercise any influence of importance on the result. The un- 

 avoidable errors in using prepared prisms will probably in most 

 cases be no less. In the above-mentioned natural prism the 

 indices of refraction a and ß were determined by determining 

 the minimum deviation. The other prism is bounded on one 

 side by the third pinacoid and on the other by a cut and 

 polished face which makes an angle of 31° 42' with the said 

 pinacoid. The refracting edge is parallel to the 6 -axis. In 

 this prism the index j', and again a, were determined by light 

 falling perpendicularly to the first-mentioned face. The following 

 values were obtained. 



a ß Y a~Y 



Red 1,6401 1,6572 1,6829 0,0428 



Yellow . . . 1,644.5 1,6609 1,6878 0,0433 



Green .... 1,6475 1,6638 1,6923 0,0448 



From these values the following axial angles are calculated 



Red Yellow . Green 



9F« = 79° 26' 77° 4' 7.5° 18' 



