404 FOURTH REPORT — 1834. 



of the inversion of the Newtonian scale of tints was observed by 

 Sir John Herschcl in some rare varieties of the same mineral. 

 The diameters of the rings, instead of contracting as the re- 

 frangibility increases, enlarge, and actually become infinite for 

 rays of mean refrangibility. Having passed through infinity, 

 they again acquire a finite value ; and diminish as the refrangi- 

 bility increases up to the extremity of the spectrum. Here, then, 

 for rays of a certain mean refrangibility the crystal is singly re- 

 fractive ; and as the double refraction changes its character in 

 passing through zero, the crystal is jwsitive for the rays of one 

 end of the spectrum, and negative for those of the other *. This 

 singular phenomenon is accounted for on the principles of Fres- 

 nel's theory by supposing that the elasticity increases, with the 

 length of the wave, faster in the direction of the axis of the cry- 

 stal than in the perpendicular direction ; so that the difference 

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

 spectrum, negative for those of the other, and vanishes at some 

 intermediate 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 posi- 

 tion of their poles, and the colours ai'e 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 manj' crystals, — such as 

 analcime and some varieties of apophyllite. The complicated 

 arrangement of the coloured bands which these substances dis- 

 play in polarized light, proves them to consist of several distinct 

 portions, possessing different optical properties ; and the phe- 

 nomena indicate relations among the molecular forces, and prin- 

 ciples 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 Brewsterf . 



* Cambridge Trans. 1821. Similar properties have been observed by the 

 same author in other crystals, as hyposulphate of lime and vesuvian. From the 

 table of tints exhibited by the latter substance it appears that the most refracted 

 of the two images is the least dispersed. 



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



