ON THE PRIMITIVE FORM OF CRYSTALS. 337 



XXIII. — Additional Observations on the Con- 

 nection between the Primitive Forms of Mi- 

 nerals and the Number of their Axes of 

 Double Refraction. 



By David Brewster, LL. D. F. R. S. Lond. and 

 Sec. R. S. Edin. 



(Read 5th August 1820.) 



In a paper printed in another part of this volume* 

 I have pointed out the connection between the primi- 

 tive forms of Crystals, as determined by Haiiy, and 

 the number of their axes of double refraction. 



In attempting to assign a reason why particular 

 forms should be distinguished by a particular num- 

 ber of axes, I found that the only exception to the 

 generality of the principle existed in the case of 

 the Eight Prism with a square base, which, in re- 

 ference to the general hypothesis, ought to have 

 had only one axis of double refraction and pola- 

 risation. As most of the crystals, however, to 

 which Haiiy assigned this as the primitive form, 



VOL. III. Y 



