140 



ON CIRCULAR POLARISATION, AS EXHIBITED IN THE 



fact seemed to confirm the supposition that the colours were 

 not dependent upon crystallisation. 



My attention was particularly directed to these phenomena, 

 (which we shall distinguish by the name of* Circular Polarisa- 

 tion,) in consequence of having discovered distinct traces of 

 them in crystals with two axes. The tints appeared at the 

 poles of the resultant axes, and the same crystal seemed to 

 unite the properties of both the varieties of rock-crystal. 

 Another specimen of the same mineral was entirely destitute 

 of these tints, so that I could not avoid drawing a conclusion 

 opposite to that of Biot, and of supposing, that, in this case at 

 least, crystallization had some share in the production of the 

 tints. This conclusion received no slight confirmation, 

 when I discovered the double system of rings in crystallised 

 sugar, and found that they exhibited none of the phenomena 

 of circular polarisation, although these phenomena are finely 

 developed in a solution of that substance. 



If the property of producing circular polarisation, were es- 

 sential to the particles of silex, it is not easy to understand 

 why it is not exhibited, to a certain degree, by all the siliceous 

 substances, and particularly by Opal and Tabasheer, the for- 

 mer of which contains above 90 per cent, of that earth. 



The great resemblance of Amethyst to Quartz, both in crystal- 

 lisation and chemical composition, induced me to examine it 

 with particular care. Werner had boldly attempted to 

 draw a distinction between these minerals, which the less 

 practised eye of other mineralogists had confounded ; and it 

 will be considered as no ordinary proof of his wonderful saga- 

 city, that this distinction has been fully confirmed by the op- 

 tical results which it is the object of the present paper to de- 

 scribe. 



Having procured above 60 Amethysts, principally from the 

 Brazils, some of which were lilac, others yellow, others green, 



and 



