486 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 
medium in the second position might be dispensed with, the medium being sensibly 
transparent. Sometimes it is useful to have recourse to analysis through a doubly 
refracting prism, or a rhomb of calcareous spar. In this way true internal dispersion 
may often be detected in a fluid which is actually muddy, in which case, were the 
effect of the incident light observed as a whole, the true would be masked by the 
enormous quantity of false dispersion which such a medium would offer. 
46. The fluids obtained by treating the leaves and other parts of plants with 
alcohol or hot water are almost always sensitive, so far as I have observed. The 
solutions in water presently ferment, and are frequently highly sensitive in the early 
stages of fermentation ; they are usually more or less sensitive in all stages. Different 
kinds of fungus furnish very sensitive solutions. When aqueous solutions become 
muddy by decomposition, other clear and often highly sensitive liquids may be 
obtained from them by various chemical processes. Port and sherry are decidedly 
sensitive. In such cases the fluid is a mixture of several substances, of which 
some may be sensitive and others insensible. When vegetable substances are 
isolated they are frequently insensible, or else so very slightly sensitive when examined 
under great concentration of the highly refrangible rays, that it is quite impossible 
to say whether the sensibility thus exhibited may not be due to some impurity: thus, 
several solutions containing sugar, salicine, morphine, or strychnine were found to 
be insensible. A solution of veratrine in alcohol proved to be sensitive in a pretty 
high degree, dispersing internally a bluish light. Sir David Brewster has 
remarked that a solution of sulphate of strychnine in alcohol dispersed light after it 
had stood for some days. This observation I have verified with reference to true 
dispersion, which the solution exhibits, though not very copiously, after it has been 
made some time. There can be little doubt that the sensitive principle in this case 
is not strychnine, but some product of its decomposition. I now come to some 
instances of internal dispersion which are far more striking. 
Solution of Leaf- Green in Alcohol. 
47 . It was in this very remarkable fluid that the phenomenon of internal dispersion 
was first discovered by Sir David Brewster, while engaged in researches relating to 
absorption. The character of the internal dispersion of a solution of leaf-green is no 
less remarkable than the character of its absorption. On account of the close con- 
nexion which seems to exist between the two phenomena, it will be requisite first to 
say a few words about the latter. 
When green leaves are treated with alcohol, a fluid is obtained which is of a 
beautiful emerald-green in moderate thicknesses, but red in great thicknesses, and 
which has a very remarkable effect on the spectrum. A good number of the following 
observations on the internal dispersion of leaf-green were made with a solution 
obtained from the leaves of the common nettle, by first boiling them in water and 
then treating them with cold alcohol, the leaves having previously been partially 
