0. Andersen — Avenlurine Feldspar. 



359 



By considering fig. 3 we infer easily how the angle p is cal- 

 culated from the angles i and r of any rays.* 



P = 



% + r 



sin i 



sint 

 n 



sin r 



sin r 

 n 



n is the mean refractive index of the feldspar. 



In order that rays falling on a certain surface after the 

 reflection from the lamellae shall emerge through the same sur- 

 face, the angle p of the lamella? must not exceed the angle of 

 total reflection for feldspar against air. This is easily seen in 

 fig. 4, which represents a feldspar containing a lamella of 



Fig. 4. 



angle p = 40°, approximately the angle of total reflection for 

 oligoclase. 



For lamellae parallel to the surface of the feldspar, the re- 

 flected rays will, of course, coincide with the rays reflected 

 directly from the surface. Such lamellae do not, therefore, 

 produce the same brilliant aventurization as lamellae of medium 

 angles p, because the colored light reflected from the lamellae 

 will be blurred by the white light reflected directly from the 

 surface of the feldspar. 



We may now consider a case where the light rays pass in 

 through one face and after the reflection from the lamellae 

 pass out through another face. In the case illustrated in fig. 5, 

 AB and A (7 represent the two cleavage faces (001) and (010) of 

 an oligoclase and EFz, lamella oriented parallel to (021) ; that is 



*See E. Eeusch, loc. cit., p. 401, where many of the optical problems are 

 discussed in detail. 



