WITHIN SOLID AND FLUID BODIES. 113 
tum 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 Joun HerscueL with the very same variety of 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, | 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 Joun HerscHEL 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 Joun HEeRscHEL 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 which 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 colour 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 Joun Herscuent describes the epipolic dispersion of this solution as 
“ occupying a very narrow parallelogram, having a breadth of about a 50th of 
* Philosophical Transactions, 1845, p. 143. 

