142 ON CIRCULAR POLARISATION, AS EXHIBITED IN THE 



sation, or rather where these tints were extinguished by the 

 opposite action of the two adjacent veins *. 



In many specimens of Amethyst, these veins are distributed 

 with great irregularity, and sometimes they are so extremely 

 thin, that the two circularly polarising structures almost entire- 

 ly disappear, and leave the crystal with the power of producing 

 a system of rings with the black cross distinctly traversing 

 them. In those specimens where the circular tints are nearly 

 extinguished, the amethyst exhibits, in the most distinct man- 

 ner, two resultant axes, inclined to one another between three 

 and. four degrees. 



The black hyperbolic branches appeared, as is usual in cry- 

 stals with two axes, in an azimuth of 45°, and different tints, 

 analogous to those of absorbing crystals, were seen within and 

 without these branches. The tints between the convex summits 

 of the hyperbolic branches, were sometimes of a deep purple^ 

 and in other specimens of a pink hue ; while the tints within 

 thesame concave summits, were of a slaty blue or of a reddish 

 white colour. The plane of the resultant axes was always per- 

 pendicular to the radius of the sector which exhibited the two 

 axes. 



Phenomena similar to those which have been described, 

 are seen likewise in the olive-coloured amethysts, and in those 

 which are colourless like quartz ; and we are therefore entitled 

 to conclude, that the amethyst combines in itself both the 



structures 



• If we consider Circular Polarisation as having its origin in a deviation from 

 the usual laws of crystallisation, the parts of Amethyst corresponding to the Mack 

 fi'inse may be regarded as produced under the influence of the usual laws, while, 

 durino- the formation of the opposite veins, between which it is interposed, the 

 crystallisation was subject to the unusual laws differently related to the axis, ac- 

 cording as the polarisation is direct or retrograde. 



