Dr. Brewster on the laws of polarisation , &c. 255 
ception is removed, as he has since found that this mineral 
has for its primitive form an irregular octohedron. 
In the paper already quoted, I have described the leading 
optical properties of this class of crystals ; but I have since 
observed several new phenomena which throw additional 
light on the subject. In the examination of thirty very fine 
cut diamonds of great size and value, I found several which 
had no action whatever upon light. Many of them pola- 
rised a fine sky blue tint of the first order, with a comple- 
mentary straw yellow, in whatever position they were held ; 
and one exhibited a succession of five or six tints, which 
were not the exact tints of Newton's scale, but similar to 
those seen near the resultant axes of crystals. In a diamond 
with natural faces, which has the form of a fine regular 
octohedron, there were no indications whatever of the pola- 
rising structure. In two specimens of leucite, broad segments of 
the coloured rings were developed when the light was trans- 
mitted in different directions ; and blende and analcime, even at 
thicknesses less than ~ of an inch, displayed a considerable 
action upon polarised light. 
M. Biot has endeavoured to account for the absence of 
polarisation and double refraction in this class of crystals 
by remarking— “ that the phenomena of the attractive class 
are explicable by a prolate ellipsoid and those of the re- 
pulsive class, by an oblate ellipsoid ; and that the sphere, 
forming a passage from one of these limits to the other 
constitutes a sort of neutral state, and corresponds to those 
crystals which, crystallizing in the regular octohedron or 
the cube, are destitute of the property of double refraction."* 
* Traits de Pbysiqut, tom. iv. p. 349, Paris, * 816. 
