MINRRAl,OfiirAI, NO'IF^ 



-ANhEKSON. 



11 



bnt it was fonml that the ti^iire fatie.s were too small to yield itiiv 

 vertectioii. Another immeisioii oF twenty niiiinteK resulted in hut slight 

 chanfife, although the residuals were reduced in ai'ea. The ejystal was 

 then acted on I'or thiity niinutes with llie disappointintr result that all 

 regularity was desti'oyed, the inarkings being now c'onclif)idal, resembling 

 thumb markings, and quite devoid ot symmetry. It was observed that 

 eei'tain depressions running in irregular directions had been developed, 



Opfifid CharacfHrii. — In a section cut parallel to the base of a oi-ystal 

 t'lom Heffeinan's Mine there is a central core which is dark between 

 crossed nicols in parallel polarised light; this is surrounded by a striated 

 zone, the stiia? parallel to the faces « and )u and consisting of thin doubly 

 refracting lamella? alternating with thicker bands of uniaxial material. 

 These thin lamellae do not extinguish simultaneously, but each parallel 

 set has straight extinction. In convergent light the interference tigure is 

 distorted, mainly uniaxial, although the cross opens out sensibly in 

 certain areas, particularly in the striated peripheral layers. These 

 observations are in substantial agreement with those of Bertrand^'' and 

 Kohlmann^^ on Muzo and Brazilian beryl respectively. 



The indices of refraction for sodium light were determined by the 

 method of minimum deviation, two alternate faces of )ii forming the 

 refracting prism. As might be expected from the lack of optical 

 homogeneity disclosed by a basal section the signals were not very sharj), 

 but concoidant results were obtained in four independent measurements, 

 as set fortli in the followins: table: — 



The average of these measurements gives « n= 1-5640, ta = 1"56S5 ; 

 thus the birefringence = '0045. Compared with previous determinations 

 these values are inclined to be low, and accepting Ford's general isation^-'' 



la Bertrand— Bull. Soc. Fr. Mm., ii., 1879. p. :51. 



>-• Kohlmanu— Neiie.s .lahrh. Miu., Beil.-B<l. xxv., IVMrT. p. 179, 



in For.l— .\mer. Journ. Sci.. xxx., 1910, pp. 128-1:50, 



