OPTICAL STRUCTURE OF THE AMETHYST. 143 



structures of the two optical varieties of quartz ; that these two 

 structures are disposed in plates parallel to the axis of the 

 prism ; that these plates are inflected into various forms, and 

 that they modify each others action, and are sometimes so mi- 

 nute as to destroy the circular tints which each of them would 

 have produced separately. 



The lilac tints, from which the amethyst derives its peculiar 

 colour, and its value as a precious stone, generally (though 

 not always) reside in the veined part of the specimen ; but 

 when the colour is removed by heat, neither the veins nor 

 their optical actions suffer any change. 



In some specimens I have found the red colouring matter 

 arranged in veins corresponding with the dark spaces where 

 the two structures destroy one another. 



This phenomenon is finely seen in the amethyst represent- 

 ed in Fig. 1. which has the appearance shewn in Fig. 2. when 

 narrowly examined with common light, and also with pola- 

 rised light, the colour of the veins varying in different posi- 

 tions, according to the quantity and nature of the tint, ab- 

 sorbed in different azimuths *. In order to explain the distri- 

 bution of the colouring matter in this and similar specimens, 

 let a, a, a, a, a, a. Sec. Fig. 3. be the black lines which separate the 

 two structures, and let b, b, b, b', b, b', &c. be the lines where 

 the one structure has begun to affect the tint of the opposite 

 structure, then the colouring matter begins at the line b, b, b, 

 and gradually increases till it is a maximum at a a a, from 

 which it again diminishes, and becomes a maximum at b', b'b\ 

 and so on, increasing and diminishing in a similar manner. 

 These tints vary in different azimuths, being sometimes purple 



in 



• See Philosophical Trcm/iactions, 1819, p. 11, 



