356 



0. Andersen — Aventurine Feldspar. 



lamellae in space. It was then possible to identify the same 

 set of lamellae in the second section on which the other angle 

 cj) was to be determined. 



A more detailed description of the microscopic measurements 

 is unnecessary, especially since the method was only used in 

 the preliminary work and in cases where the angles p of the 

 lamellae were too large to be measured with the goniometer. 



Fig. 1. 



Some of the measurements will be recorded in connection with 

 the description of the specimens. 



Remarks on the Optics of Aventurine Feldspars. 



Before describing the measurements with the goniometer a 

 brief general discussion of the course of light rays in aven- 

 turine feldspars will be in place. We consider only rays in 

 the main reflection plane, the plane perpendicular to the sec- 

 tion lines of the lamellae, as only such rays are used in the 

 measurements. 



In fig. 1, A B represents the surface of a cleavage piece con- 

 taining a reflecting lamella E F G H. The plane of the 

 drawing is perpendicular to the section line of the lamella. 

 The lamella, drawn at an angle p of approximately 21°, has 



