( in ) 



XIII. — On the Decomposition and Dispersion of Light within Solid and Fluid 

 Bodies. With a Plate. By Sii- David Brewster, K.H., D.C.L., F.R.S., and 

 V.P.R.S. Edin. ' .• 



(Read 2d February 1846.) 



Hauy*, and other mineralogists, observed the two colours which are visible 

 in several varieties of fluor-spar. He regai-ded the two tints as complementary, 

 and explained them, as he did every other analogous phenomenon, by a reference 

 to the colours of thin plates. In describing a species of dichroism noticed by Dr 

 PROUTf in the purpurates of ammonia and potash, Sir John Herschel ascribes 

 the green reflected light i: " to some peculiar conformation of the green surfaces 

 producing what may be best termed a superficial colour, or one analogous to the 

 colour of thin plates, and striated or dotted surfaces." And he adds — " A remark- 

 able example of such superficial colour, differing from the transmitted tints, is 

 met with in the green fluor of Alston Moor, which, on its surfaces, whether na- 

 tural or artificial, exhibits, in certain lights, a deep blue tint, not to be removed 

 by any polishing." 



Having, many years ago, found the same property in the Derbyshire fluor- 

 spars, I was led to study it with particular attention ; and, in 1 838, 1 communicated 

 the results of my observations to the British Association at Newcastle. $ In every 

 specimen in which the colour in question exists, I found it to arise from internal, 

 and not from superficial reflexion. In an extensive series of experiments on the 

 absorption of light by the aqueous and alcoholic solutions of the colouring matter 

 of plants, I found this property of internal dispersion in thirty or forty of these 

 solutions. The most remarkable of these M^as the alcoholic solution of the colour- 

 ing matter of the leaves of the common laurel. At first its colour is a bright 

 green, afterwards changing into a fine olive colour ; but in all its stages it dis- 

 perses light of a brilliant blood red colour, which forms a striking contrast with 

 the transmitted tint. After a long exposure to light, the transmitted tint almost 

 whoUy disappears, while the dispersed light retains its red colour. || Another 



* Traite de Mineralogie, torn, i., p. 512, 521. 



t Philosophical Transactions, 1818, p. 424. 



X Treatise on Light, art. 1076. 



§ See Keport of the Eighth Meeting, and Trans, of Sections, p. 10-12. 



II I shewed this experiment in 1836, at Lacock Abbey, to Mr Fox Talbot, and several mem- 

 bers of the British Association. At the meeting of the British Association at Manchester, in 1842, 

 a friend handed to me, in the sectional meeting, a " solution of stramonium in ether," which 

 VOL. XVI. PART II. 2 F 



