PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF EIGHT. 485 
chlorophyll, or one of its modifications- The light transmitted by the fluid exhibited 
on prismatic analysis the absorption band in the red which is so characteristic of that 
substance. 
The colour of the solution was a pale brownish yellow ; it would no doubt have 
been still paler, and perhaps nearly colourless, had the sensitive principle to which 
the green dispersion was due been present in equal quantity but in a state of purity. 
As it was, the fluid was pale enough to exhibit well, when poured into a test tube and 
held in front of a window, a narrow arc on the side of the incident light, like sulphate 
of quinine, only in this case the arc was green instead of blue. 
Frequency of the occurrence of true internal dispersion having the same general 
character as that which takes place in the cases above described. 
44. If we except the red dispersed beam produced by red rays in the crystal of 
fluor-spar and in the stramonium extract, a strong similarity may be observed in the 
mode of internal dispersion which takes place in the cases hitherto described. As 
the refrangibility of the incident light continually increases, the rays are at first in- 
active. At a certain point of the spectrum, varying according to circumstances, the 
true dispersion begins to be sensible, but is faint at first. After remaining faint for 
some distance it presently becomes more copious. It remains very conspicuous 
through the whole of the violet and beyond, and then gradually dies away. It con- 
sists at first of light of comparatively low refrangibility, and then new colours in the 
order of their refrangibility enter into it. Frequently the greater part of the change 
of prismatic composition takes place while the dispersed light is very faint, so that 
practically speaking we may almost say that the tint is uniform. Sometimes, when 
the dispersion just commences, the dispersed light is nearly homogeneous, and has a 
refrangibility so nearly equal to that of the active light that the beams due to true 
and false dispersion can hardly be separated. 
45. Now this, so far as I have observed, is much the commonest kind of true in- 
ternal dispersion, although sometimes the phenomenon presents very striking singu- 
larities. In the paper in which Sir David Brewster first announced the discovery 
of internal dispersion, he remarks “ that it is a phenomenon which occurs almost 
always in vegetable solutions, and almost never in chemical ones or in coloured 
glasses*.” For my own part, I have rarely met with a vegetable solution which did 
not exhibit more or less the phenomenon of true internal dispersion. Its existence 
may in general be easily detected in the following manner. The sun’s light being 
reflected horizontally through a lens, a deep blue glass is left in such a position as to 
intercept the light incident on the vessel containing the fluid, which is placed at the 
focus of the lens. A pale brown glass of the proper kind is then placed so as to 
intercept, first the incident, and then the dispersed light. A vessel with flat sides filled 
with a solution of sulphate of quinine would be better, and then the placing of the 
* Edinburgh Transactions, vol. xii. p. 542. 
