COLOURS OF CRYSTALLINE PLATES. 139 



rays of one end of the spectrum, and negative for those of the 

 other.* This singular phenomenon is accounted for on the prin- 

 ciples of Fresnel's theory by supposing that the elasticity increases, 

 with the length of the wave, faster in the direction of the axis of 

 the crystal than in the perpendicular direction ; so that the difference 

 of these elasticities is positive for the rays of one end of the spec- 

 trum, negative for those of the other, and vanishes at some inter- 

 mediate point. 



In biaxal crystals similar deviations take place in the magni- 

 tude of the lemniscates corresponding to the different simple 

 colours. But there is here another source of irregularity which 

 is not found in uniaxal crystals. The optic axes vary, in general, 

 with the colour ; so that the lemniscates differ also in the position 

 of their poles, and the colours are not the same in different parts 

 of the same ring. Where the optic axes belonging to different 

 colours are in different planes, as Sir John Herschel has observed 

 to be the case in borax, the irregularity produced in the coloured 

 curves is yet more striking. 



In all the preceding cases, the laws of double refraction are 

 dependent only on the direction, and are the same all throughout 

 the mass. It is otherwise, however, in many crystals, such as 

 analcime and some varieties of apophyllite. The complicated 

 arrangement of the coloured bands which these substances display 

 in polarized light proves them to consist of several distinct por- 

 tions, possessing different optical properties ; and the phenomena 

 indicate relations among the molecular forces, and principles of 

 aggregation, of which it is difficult in some cases even to form a 

 conception. These remarkable phenomena, and their laws, were 

 discovered by Sir David Brewster.f 



When a polarized ray traverses a plate of Iceland spar, beryl, 

 or almost any other uniaxal crystal, in the direction of its axis, 

 it suffers no change of any kind ; so that when the emergent ray 

 is analyzed by a double-refracting prism, the two pencils into 

 which it is divided are colourless, and one of them vanishes when 

 the principal section of the prism is parallel or perpendicular to 



* Cambridge Trans. 1821. Similar properties have boon observed by the same 

 author in other crystals, as hyposulphate O f limo and vesuvian. From th 

 tints exhibited by the latter substance it appears that the most refra 

 images is the least dispersed. 



t Edin. Trans., vols. ix. & x. 



