112 SIR DAVID BREWSTER ON THE DECOMPOSITION AND DISPERSION OF LIGHT 



very remarkable example of internal dispersion, pointed out to me by Mr Schunck, 

 is exhibited in an allvaline, 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 c/reeji colour. 



Since these experiments were made, my attention has been called to two 

 interesting papers by Sir John Herschel, in the last part of the Philosophical 

 Transactions; the one on a cane of svperjicial colour presented by a homogeneous 

 liquid internally colourless, and the other on the epijwlic (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 colom-less 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 hluish-yellotv varieties from Derbyshu-e. In order 

 to obsei"ve 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 ABC, 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 hriyht 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 tliat 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. 



