WITHIN SOLID AND FLUID BODIES. 113 



turn there was no dispersion at all ; this was followed by a narrow stratum, which 

 dispersed a bright whitish light ; then succeeded a stratum of non-dispersing fluor, 

 and alternately dispersing and non-dispersing strata, scattering the fine blue light 

 which has already been mentioned. 



These results, which I have shewn to different persons, are incompatible 

 with those obtained by Sir John Herschel with the very same vai-iety oi fluor- 

 spar. He regards the blue dispersed light as strictly an epipolic or superficial 

 tint, — so superficial, indeed, " that it might be referred to a peculiar texture 

 of the surface, the result of crystallization, were it not that it appears equally 

 on a surface artificially cut and polished." * Were I to hazard a conjecture 

 respecting the cause of this difference in our results, I would ascribe it to the 

 different degrees of light in which the observations were made. While I used a 

 condensed beam of the sun's light. Sir John Hekschel seems to have employed 

 chiefly the ordinary light of day. In studying the phenomena in the solution of 

 quinine, he " exposed it to strong day-light or sunshine ;" and in another expe- 

 riment, which pre-eminently required a powerful illumination, he " directed a 

 sunbeam downwards on the surface, by total reflection from the base of a prism," 

 which was in reality inferior to the ordinary sun's light. In the case of fluor- 

 spar, however, he states that the epipolic colour is seen in perfection when " ex- 

 posed to daylight at a window." In such a feeble light I could not have seen the 

 phenomena I have described, and it is owing chiefly to the intensity of the light 

 which I employed, that I have been enabled to place it beyond a doubt that the 

 blue light dispersed by fluor-spar is reflected from every part of the interior of 

 the crystal, and is not produced by any action either strictly or partially super- 

 ficial, or solely by any stratum near the surface. 



Sir John Heeschel mentions, that the green fluor-spar of Alston Moor is 

 the only solid in which he has observed an epipolic tint. It is the only mineral 

 in Avhich I have found an internal dispersion, excepting, of course, the minerals 

 which exhibit the analogous phenomena of opalescence and chatoyance ; but I have 

 found several glasses which possess it, one in particular of a yellow colour, which 

 disperses a brilliant green light, and another of a bright pink colour, which also dis- 

 perses a green light, and a third of an orange colour, which disperses rays of a 

 whitish green colour. In these cases, the glass has a decided coloiu* of its own ; 

 but I have found many specimens, both of colourless plate and colourless flint 

 glass, which disperse a beautiful green light. 



2. On the Internal Dispersion of the Solution of Sulphate of Quinine. 

 Sir John Herschel describes the epipolic dispersion of this solution as 

 " occupying a very narrow paraUelogi'am, having a breadth of about a 50th of 



* Philosophical Transactions, 1845, p. 143. 



