192 DR BREWSTER on a New Species of Double Refraction, 



a correct notion of the phenomena. In Fig. 10. I have repre- 

 sented the Cube, seen in perspective; and in Figure 11. the 

 Cube, projected on a plane perpendicular to one of its diagonals. 

 Figure 9. represents the Icositetrahedron in perspective ; and 

 Figure 12. the same solid projected on a plane perpendicular to 

 one of its axes. 



One of the most important results of these experiments, is 

 the singular distribution of the doubly refracting force, not mere- 

 ly in the crystal considered as a whole, but in each of the sepa- 

 rate pentahedrons which compose it. In all other crystals in 

 which the laws of double refraction have been studied, the axis to 

 which the doubly refracting force is related has no fixed locality 

 in the mineral. It is a line parallel to a given line in the pri- 

 mitive form, and every fragment of a crystal, however minute, 

 possesses this axis, and all the optical properties of the original 

 crystal, however large. The property of double refraction, in 

 short, in regularly crystallised substances, resides in the ulti- 

 mate particles of the body, and does not depend upon the mode 

 in which they are aggregated to form an individual crystal. 



In Analcime, on the contrary, we have planes of no double 

 refraction, having a definite and invariable position, and we may 

 even extract a portion of each separate pentahedron which has 

 no axis at all. 



Nor has the doubly refracting structure of Analcime any re- 

 lation to that of composite crystals, such as the Bipyramidal Sul- 

 phate of Potash*, which consists of several individual rhomboidal 

 prisms, beautifully combined to form a regular geometrical solid, 

 or that still more complicated mineral Apophyllite, where an in- 

 dividual crystal with one axis is symmetrically united with se- 

 veral individual crystals with two axes, so as to constitute a re- 

 gular crystal f . In these, and other cases, each individual crystal 



* See Edinburgh Philosophical Journal, vol. i. p. 6. 



+ See Edinburgh Philosophical Journal, vol. i. p, 1. ; and Edinburgh Tranz- 

 i vol. ix. p. 317. 



