0. Andersen — Aventurine Feldspar. 361 



the refraction at the surface A B the red rays follow the 

 course p h' p" a" and the violet rajs the course p hp' a'. The 

 reflection from the lamellae of aventurine feldspars will thus, 

 in general, be accompanied by a color dispersion of the light. 

 The magnitude of the dispersion will depend on the angle i 

 of the incident rays, the angle p of the lamellae and the specific 

 power of dispersion of the feldspar. As this power of disper- 

 sion is low with feldspars the actual color dispersion in aven- 



Fig. 6. 



B 



turine feldspars will always be insignificant. For lamellae 

 parallel to the surface (p — 0) there will be no dispersion. 



GopJometric Measurements. 



For the goniometric measurements cleavage pieces with 

 smooth faces of the forms (001) and (010) and showing distinct 

 aventurization on both faces were selected. The faces gener- 

 ally measured from 2 X 2 mm to 10 X 10 mm . 



The measurements were made with a Goldschmidt two- 

 circle goniometer in the following way : The cleavage face to 

 be used as equator was first adjusted parallel to the vertical 

 circle of the goniometer. The cleavage piece could then be 

 rotated round the horizontal axis without changing the vertical 

 position of the face. 



In cleavage pieces composed of polysynthetic twins the 

 single twinning lamellae were sometimes broad enough for in- 

 dividual adjustment and contained a sufficient number of 



