480 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 
two independent variables, namely, the lengths of the path before and after dispersion; 
whereas, had the light been merely reflected from coloured particles held in suspen- 
sion, the effect of absorption would have been a function of only one independent 
variable, namely, the length of the entire path within the medium. 
29. When false dispersion abounds in a fluid, it may be detected at once by the 
eye, without having recourse to any of the characters already mentioned whereby it 
may be distinguished from true dispersion. When a fluid is free from false dispersion 
it appears perfectly clear, when viewed by transmitted light, although it may be 
highly coloured, and may even possess to such an extent the property of exhibiting 
true internal dispersion as to display, when properly viewed, a copious dispersive 
reflexion. On the contrary, when false dispersion abounds, the fluid, if not plainly 
muddy, has at least a sort of opalescent appearance when viewed by transmitted light, 
which, after a little experience, the eye in most cases readily recognises. In viewing 
the phenomenon of dispersive reflexion, as exhibited in a fluid, it might be supposed 
that the fluid was water, or else some clear though coloured liquid, holding in suspen- 
sion a water colour in a state of extreme subdivision. But on holding the fluid 
before the eye, so as to view it by transmitted light, or rather view a bright well- 
defined object through it, the illusion is instantly dispelled. The reason of this 
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. 
Decoction of the Bark of the Horse- Chestnut (yEscuIus hippocastanum). 
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. Ha^dng 
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 dilFerence 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 Gi^H, the 
excessively faint dispersion which was exhibited earlier being left out of consider- 
ation, whereas in a decoction of the bark of the horse-chestnut, diluted so as to be 
