144 ON CIRCULAR POLARISATION, AS EXHIBITED IN THE 



in the plane of primitive polarisation, and pale-red in a plane 

 at right angles to it. 



In another amethyst, shewn in Fig. 4. the portion A gave 

 the retrograde tints, and B the direct tints : the direct veins 

 were lilac ; the retrograde veins brownish-red ; and the dark 

 lines yellowish- white. 



In other crystals the colouring matter affects the largest 

 masses of the structure, such as those left white in Fig. 1. which 

 separate the veined sectors. In a very interesting speci- 

 men, shewn in Fig. 5. the lines AE, BF, ^DG, which divide 

 the hexahedral crystal into triangular prisms, are distinctly 

 seen by common light ; and one of these prisms, BCD, is 

 alone tinged with the red-colouring matter. Upon exposing 

 this ci'ystal to a polarised ray, I found that the sector BCD, 

 consisted of two opposite structures a, 6, separated by the 

 curved vein m n, where their opposite actions were in equi- 

 librio ; and hence it follows, that the colouring matter affected 

 all the three structures of the specimen. This subdivision of 

 the crystal into hexahedral prisms, seems to indicate that the 

 dodecahedron may be formed by two intersecting rhomboids,., 

 to which the two structures may be related. 



In a large white amethyst, tinged with masses of yellow, 

 I found that the yellow-colouring matter resided in three une- 

 qual sectors. A, B, C, Fig. 6. all the rest of the crystal, consist- 

 ing of narrow veins of the opposite structure, which became so 

 minute as to destroy one another almost entirely in the sector 

 D, which exhibited in the highest perfection the two axes and 

 the hyperbolic branches. The sectors A and C were, as it 

 were, expansions of direct veins, while B was the expansion 

 of a retrograde vein, and B and C were separated by a dark 

 line, from which the tints ascended to a green of the second 

 order. 



Among 



