113 



to which answers 



2E ^ 41° 3'. 

 A slight dispersion is found, which appears by the fact 

 that the hyperbolas are quite slightly blue on the inside, 

 accordingly : 



The indices of refraction of the catapleiite have been de- 

 termined by total reflection on basal faces of the crystals of 

 type 1. In the crystals collected by Bernburg's expeditions, 

 which are distinguished by remarkably large, optically homo- 

 geneous parts, the surfaces are, unfortunately, not completely 

 plane; therefore the older material has been used; on account 

 of the twin formation the boundary lines of a and Ь are most 

 frequently not distinctly separated, and they often occur simul- 

 taneously; the boundary lines of c, on the other hand, is al- 

 ways very definite. As an average value of several, very little 

 differing measurements have been found for yellow light: 



a = 1-5905 

 ß =■ 1-5921 

 r = 1-6269. 

 y—a = 0-0364 ß—a == 0-0016. 



The value of у — a corresponds tolerably well to the double 

 refraction in vertical sections found by Michel-Lévt and Lacroix^), 

 viz. 0-030. For ß — a Brøgger has found 0-004. By means of 

 the compensator I have found for the Greenland catapleiite : 

 ß—a = 0-0018. 

 The far greater value found in the Norwegian mineral is 

 connected with the larger axial angle of this mineral before 

 mentioned. 



From the above indices of refraction may be calculated: 

 2F == 25° 25' 

 2E = 41° Г. 



1) Min. d. roch. p. 166. 



