O. Andersen- — Aventurine Feldspar. 357 



the actual position of the main set of lamellae that cause the 

 aventurization on (001) of the feldspars. The rays are con- 

 structed at the approximately true angles obtaining in an oligo- 

 clase (n = l - 54:). A very high refractive index («.*= about 3) 

 is assumed for the construction of the rays inside the lamella. 

 In the discussions we disregard (as we do in the figures) the 

 double refraction of the feldspar, and also an eventual double 

 refraction of the lamellae. This will simplify the problems 

 very considerably without changing their general character. 



Fig. 2. 



The course of the different rays originating from the inci- 

 dent ray a p is shown in fig. 1. It is supposed that the lamella 

 is thin enough to allow a part of the light to penetrate to its 

 lower surface G H where one ray o s proceeds into the feldspar 

 and the other is reflected in the direction o k. There will then 

 be opportunity for an interference between the two rays hj/ 

 and kp" with a path difference equal to h ok or approximately 

 the double thickness of the lamella (as the angle h o k is 

 always very small). In white light we will, therefore, see 

 interference colors in the direction p' a' (p" a") of the light 

 rays which pass out through the surface A B. 



In the following discussion we use the symbol* for the angle 

 ap n (fig. 1), the angle of incidence of rajs falling on a surface 

 (AB) and the symbol r for the angle a' j>' n', the angle at which 

 the same rays emerge from the same surface after the reflection 

 from the lamellae. 



Fig. 2 shows the relations between the angles i and r for a 

 number of rays constructed on the basis of the same properties 

 as flg. 1. (Feldspar with n = 1/54 ; lamellae with angle p = 

 21°.) The angles i and r will differ somewhat for different 

 aventurine feldspars and will especially depend on the angles 



