aia 8 aa 
XIII.—On the Decomposition and Dispersion of Light within Solid and Fluid 
Bodies. With a Plate. By Sir Davip Brewster, K.H., D.C.L., F.R.S., and 
V.P.R.S. Edin. K AE 
(Read 2d February 1846.) ; 
Havy*, and other mineralogists, observed the two colours which are visible 
in several varieties of fluor-spar. He regarded 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 
Proutt in thé purpurates of ammonia and potash, Sir Joun HeErscHEL ascribes 
the green reflected light{ “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 1838, I 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 znternal, 
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 was 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 
wholly disappears, while the dispersed light retains its red Golour.|| Another 
* Traité de Mineralogie, tom. i., p. 512, 521. 
t Philosophical Transactions, 1818, p. 424. 
t Treatise on Light, art. 1076. 
§ See Report of the Eighth Meeting, and Trans. of Sections, p. 10-12. 
|| I shewed this experiment in 1836, at Lacock Abbey, to Mr Fox Tatzor, 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. 2F 
