298          ON THE  CHANGE  OF  REFRANGIBILITY  OF LIGHT.
change of prismatic composition takes place while the dispersed light is very faint, so that practically speaking we may almost ^f|/                            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 internal dispersion, although sometimes the phenomenon presents very striking singularities. 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 mariner. 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 ,1 |                         would be better, and then the placing of the medium in the
|                         second  position   might  be   dispensed   with,   the   medium   being
. i\                         sensibly transparent.    Sometimes it is useful to have recourse to
f                          analysis through a doubly refracting prism, or a rhomb of cal-
«'/,;                               careous spar.    In this way true internal dispersion may often be
<•                                detected in a fluid which is actually muddy, in which case, were
i                               the effect of the incident light observed as a whole, the true
^1,'                              would be masked by the enormous quantity of false dispersion
;i                              which such a medium would offer.
i
^i;                              46.    The  fluids obtained by treating the  leaves and other
,|                         parts of plants with alcohol or hot water are almost always sensi-
I                          tive, so far as I have observed.    The solutions in water presently
!                          ferment, and are frequently highly sensitive in the early stages of
'I                         fermentation; they are usually more or less sensitive in all stages.
J                                                          * Edinburgh Transactions, Vol. xu. p. 542.