IX. On Circular Polarisation, as exhibited in the Optical 

 Structure of the Amethyst, with Remarks on the Distribution 

 of the Colouring Matter in that Mineral. By David 

 Brewster, LL. D. F. R. S. Lond. & Edin. * 



(Read November 15. 181 9. J 



AN plates of Rock-Crystal cut perpendicular to the axis of 

 the prism, an unusual kind of polarisation had been observed 

 in the colours seen along the axis. The phenomena were 

 subsequently analysed by M. Biot, who remarked, that in 

 some specimens of quartz, the succession of tints appeared 

 by turning the doubly refracting prism from right to left, while 

 in other specimens the same succession was developed by 

 turning the prism from left to right ; and he concluded from 

 his experiments, that the quartz impressed upon the particles 

 of light a rotatory motion, and that this property belonged to 

 the ultimate particles of silex, and was independent of their 

 mode of aggregation. 



The same species of colours was afterwards observed, about 

 the same time, by MM. Biot and Seebeck, in transmitting 

 polarised light through considerable thicknesses of some essen- 

 tial oils, and solutions of sugar and camphor ; and this new 



s 2 fact 



* The properties of Amethyst, described in this paper, were discovered in 1817; 

 and were announced to the Royal Society on the 18th January 1819. 



