^O* Sir D. Brewster on the Decompositio7i and Dispersion 



not produced by any action either stiictly or partially super- 

 ficial, or solely by any stratum near the surface. 



Sir John Herschel mentions that the green fluoi'-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 phaenomena of opalescence and chatoy- 

 ance ; but I have found several glasses which possess it, one 

 in particular of a ijelloia colour, which disperses a brillia?it 

 greeji light, and another of a bright pink colour, which also 

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

 disperses rays of a xvhitish-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 

 Qiiinine. 



Sir John Herschel describes the epipolic dispersion of this 

 solution as "occupying a very narrow parallelogram, having 

 a breadth of about a fiftieth of an inch, of a vivid and nearly 

 uniform blue colour over its whole breadth*;" but upon 

 " directing a sunbeam downwards on the surface, by total re- 

 flexion from the base of a prism, a feeble blue gleam was 

 observed to extend downwards below this vivid line to nearly 

 half an inch from the surface, thus leaving it doubtful whether 

 some small amount of dispersion may not be effected in the 

 interior of the medium at appreciable depths." By using 

 condensed solar light, this doubt is immediately removed, and 

 the phenomenon ranks itself as one of internal dispersion, 

 differing only in the law of its intensity from those which I 

 have already described. In the one the dispersible rays are 

 thrown gradually, in the other quickly, from the intromitted 

 beam, — a phaenomenon to a great extent identical with what 

 takes place in the analogous phcenomena of absorption. 



If the dispersing action of the solution were rigorously con- 

 fined to a stratum the fiftieth of an inch thick, it would have 

 followed of necessity, that " an epipolizedbeam of light (mean- 

 ing thereby a beam which has been once transmitted through 

 a quiniferous solution, and undergone its dispersing action) is 



* The best method of seeing this experiment, is to take the solution into 

 the open air, where the whole light of a blue sky can fall upon its surface. 

 I have in this way seen the blue line perfectly luminous at that stage of a 

 December twilight when there was not light enough to read by. ( con- 

 sider, therefore, the light of the sky as peculiarly susceptible of this species 

 of dispersion. 



