494 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 
about the fixed line D, the dispersed light being at first nearly homogeneous, and of 
the same refrangibility as the active light. On proceeding onwards in the spectrum, 
in observing by the fourth method, the orange beam became brighter, and yellow 
entered into it, but no colour beyond that, so that the orange and yellow beam was 
left behind by the beam of falsely dispersed light, from which it was separated by a 
perfectly dark interval. The green dispersion began about b, or a little beyond, 
coming on almost abruptly. The manner of its commencement was best observed 
by the fourth method, by holding a prism to the eye while the lens was moved 
through the spectrum. In this way it was found that on arriving at the point of the 
spectrum above mentioned, a gleam of green light shot across the dark space which 
before separated the beam of falsely dispersed light from the orange beam of truly 
dispersed light. As the lens moved on, the green dispersed light grew brighter, but 
its more refrangible limit did not seem to pass, or at least much to pass, the refran- 
gibility it had at first ; so that the green beam of truly dispersed light was almost 
immediately left behind by the beam of falsely dispersed light. The former, on being 
left behind, soon died away. 
67. We might suppose either that the red, orange and green dispersions are due to 
the same sensitive principle, or that they are produced by three distinct sensitive prin- 
ciples mixed together in the solution. The latter would appear the more probable 
supposition, to judge by the apparent want of connexion between the three disper- 
sions. This view is strongly confirmed by the following results. Some ether was 
poured on archil in the fluid state, and after being gently moved about and allowed 
to stand, a little was withdrawn without agitation. A purplish rose-coloured fluid 
was thus obtained, which was highly sensitive, exhibiting the orange and green di- 
spersions but not the red. The orange dispersion was far more copious, in proportion 
to the whole quantity of dispersed light, than had been the case with archil diluted 
with water. 
Some archil was violently agitated with ether, and after subsidence the ether was 
withdrawn. This ethereal solution was much deeper in colour than the former, and 
exhibited the red dispersion in addition to the orange and green. On adding a small 
quantity of water, and agitating, a separation, or at least partial separation, of the 
sensitive principles took place ; for the upper fluid exhibited the orange dispersion 
abundantly, but none of the red, and little or none of the green, while the under fluid 
exhibited the green and red dispersions with little, if any, of the orange. The upper 
fluid exhibited a pretty copious dispersive reflexion of reddish orange, and the under 
fluid a remarkably copious reflexion of a fine green. A similar separation, more or 
less perfect, took place in other cases, the dispersion of orange bearing to that of 
green a greater ratio in the ether than in the water. Some of the green-dispersing 
fluids thus obtained were most remarkable on account of the extraordinary copious- 
ness of the reflected green, and the strange contrast which it presented to the trans- 
mitted tint, which was a purplish red. 
