372 



0. Andersen — Aventurine Feldspar. 



-vibrating perpendicular to the plane of incidence and the trans- 

 mitted ray op'" a'" vibrating in the same plane. Now if the 

 incident ray {a p), therefore, consists of polarized light the 

 transmitted ray (p'" a'") will have its maxim um of intensity 

 for the vibration direction perpendicular to the section line of 

 the lamella and the minimum for the direction parallel to this 

 line. This is exactly the apparent pleochroism (g) > e) observed 

 on lamellae of large angles p. 



The correctness of the above explanation was proved by the 

 fact that the light reflected from lamellae of large angles p 



Fig. 12. 









a'a' 



i 



. 









>>' 



v'J^ 









//''' 





t 



H F/ 











Uw 







B 



I) 



(rays p' a', fig. 12) was found to be strongly polarized with 

 vibration direction parallel to the section lines of the lamellae. 

 This was observed in a number of sections with lamellae of 

 angles p around 75°. It is then a necessary conclusion that the 

 transmitted rays (p'" a'") must be polarized with vibration 

 direction perpendicular to the section lines. 



The apparent pleochroism in the lamellae of aventurine 

 feldspars when observed at large angles p is not, therefore, 

 due to any strong difference in absorption between the vibra- 

 tion directions <o and e (if we suppose that the lamellae consist 

 of hematite tables), but is explained, as outlined above, by the 

 polarization of rays when they fall on the lamellae at the 

 appropriate angles of incidence. 



