ON THE  CHANGE OF REFRANGIBILITY OF LIGHT.          291
difference appears to admit of easy explanation, and will be noticed further on.
30. Light will be spoken of in this paper as active when it is considered in its capacity of producing other light by internal dispersion. A medium will be said to be sensitive when it is capable of exhibiting dispersed light under the influence of light (visible or invisible) incident upon it. In the contrary case it will be called insensible.
I shall now return to the description of the appearances exhibited by some of the media most remarkable for their sensibility.
•11
Decoction of the  Bark of the Horse-Chestnut (JEsculus  hippo-
castanum).
31. In Sir John Herschel's second paper it is stated that esculine possesses in perfection the peculiar properties which had been found to belong to quinine. Having tried without success to procure the former alkaloid*, I was content to let this substance pass, till I found how admirably a mere decoction or infusion of the bark of the tree answered for all purposes of observation.
This medium is even more sensitive than a solution of sulphate of quinine, and disperses like it a blue light. The description of the mode of dispersion in the latter medium will apply in almost all points to the former: the principal difference consists in the circumstance that in the horse-chestnut solution the dispersion begins earlier in the spectrum than in the solution of quinine. In a solution of sulphate of quinine of convenient strength, we have seen that the dispersion came on at about G^H, the excessively faint dispersion which was exhibited earlier being left out of consideration, whereas in a decoction of the bark of the horse-chestnut, diluted so as to be of a convenient strength, it came on a little before G. This explains the reason of an observation of Sir David Brewster's, who has remarked that "a beam of light that has passed through the esculine solution disperses blue light, but not copiously, when transmitted through the quinine solution ; but the beam that has passed through quinine is copiously dispersed when transmitted through esculine f."
* [It is a glucoside.]
t Philosophical Magazine, Yol. xxxn. (June 1848), p. 406.
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