112 SIR DAVID BREWSTER ON THE DECOMPOSITION AND DISPERSION OF LIGHT 
very remarkable example of internal dispersion, pointed out to me by Mr Scuunckr, 
is exhibited in an alkaline, or in an alcoholic solution of a resinous powder pro- 
duced from orcine by contact with the oxygen of the air. Its colour by transmit- 
ted light is reddish brown, and the light which it disperses is of an exceedingly 
rich green colour. 
Since these experiments were made, my attention has been called to two 
interesting papers by Sir Joun HeRscHEL, in the last part of the Philosophical 
Transactions; the one on a case of superficial colour presented by a homogeneous 
liquid internally colourless, and the other on the epipolic (or superficial) dispersion of 
light; and as these papers contain results incompatible with those which I had 
previously published, I found it necessary to resume the investigation of the 
subject. 
The two papers now referred to are chiefly occupied with a description of the 
phenomena of coloured dispersion, as exhibited in a diluted solution of sulphate of 
quinine in weak sulphuric acid. Owing to the solution being nearly colourless by 
transmitted light, the general phenomenon is very beautiful. The line of bright 
blue light dispersed by the stratum of fluid immediately beneath the surface of 
incidence, and about the 50th of an inch thick, appears to be confined to that stra- 
tum, and it is in this respect only that the phenomenon differs from that which is 
exhibited by fluor-spar and the vegetable solutions which I have mentioned. 
1. On the Internal Dispersion of Fluor-Spar. 
There are many varieties of fluor-spar in which no dispersion of the intro- 
mitted light takes place. It does not exist in the yellow, red, and bright blue va- 
rieties which I have examined. It occurs chiefly in the green fluor from Alston 
Moor, and in several pink, and bluish-yellow varieties from Derbyshire. In order 
to observe the phenomena of dispersion most distinctly, I transmit a condensed 
beam of the sun’s light through the specimen, when partially covered with black 
wax or black velvet. In some specimens, the intromitted beam is partially dis- 
persed in a fine blue tint from every part of the solid which it traverses; but in 
other specimens, which are composed of strata of different colours, parallel to the 
faces of the cube, a very different and a very instructive phenomenon is displayed. 
The intromitted beam A BC, Fig. 1, Plate V., is crossed with bands of dispersed 
light of different colours, and of different intensities. In one case, a pink light was 
dispersed from the stratum close to the surface of incidence; from the next stra- 
dispersed a bright green light. I described the phenomenon to the meeting, and it is noticed in the 
Transactions of the Sections, p. 14. Upon making the solution myself, I cannot obtain the same 
tints, either from the stalk or the dried leaves of the plant. The solution of the leaves disperses a 
brilliant red tint, like that mentioned in the text. The solution put into my hands must, therefore, 
have been one of the seeds of stramonium, or of some other substance possessing internal dispersion 
in a high degree. 
