492 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 
spectrum at which the incident light penetrates to comparatively great distances 
into the fluid before it is absorbed. The oblique position observed in the first mini- 
mum is readily explained by considering that the illumination at any point of the 
field of view depends conjointly upon the activity of the incident light, which is 
a function of its refrangibility, and upon the fraction of the incident light left 
unabsorbed, which last is a function both of the refrangibility and of the distance 
from the first surface. 
61. It seems worthy of remark, that while the quantity of dispersed light is liable 
to fluctuations having an evident relation to the bands of absorption which occur 
throughout the spectrum, the quality of the light dispersed, as regards its refrangi- 
bility, appears rather to have reference to the intense absorption band No. 1. 
Extract from blue leaves of the Mercurialis perennis. 
62. The juice of this plant has the property of turning blue by exposure to the air. 
Some leaves and stalks which had turned blue were treated with alcohol, and a green 
fluid was thus obtained much resembling in colour the ordinary solutions of leaf- 
green, but I think of a rather bluer green than usual. In its mode of absorption, too, 
it much resembled ordinary solutions of leaf-green, to which substance no doubt the 
greater part of its colour was due. Its internal dispersion however was very peculiar, 
for it dispersed a copious orange in place of a blood red like the extracts from fresh 
green leaves in general, those of the Mereurialis perennis included. On analysis the 
dispersed beam was found to consist chiefly of a red band, similar to that which 
occurs in solutions of leaf-green, and of a yellow or orange and yellow band, a good 
deal brighter than the former, from which it was separated by an intervening dark 
band. When the fluid was examined by the second method, it was found that 
the yellow dispersion was produced principally by the brightest part of the spectrum. 
After a considerable time the fluid lost its fine green colour, as is very often the case 
with solutions of leaf-green, and became yellowish brown, but the red and yellow 
dispersions still continued. 
When the fluid was examined by the fourth method, it was found that the red 
rays dispersed a red, just as in a solution of leaf-green. The additional dispersion 
which was so conspicuous in this fluid began almost abruptly about the fixed 
line D. When it was first observed, the refrangibility of the orange dispersed light 
could hardly, if at all, be separated from that of the active light. As the lens moved 
on, the orange beam rapidly grew brighter, and yellow entered into it ; and now it 
was easy to see that the beam of falsely dispersed light lay at its more refrangible 
limit. The orange and yellow dispersed beam was brightest at about DfE; but 
though it decreased in intensity it could be traced far beyond that point, in fact, 
throughout the spectrum. 
63. I have generally found that when a copious dispersion commences almost 
abruptly at a certain point of the spectrum, it is followed by a band of absorption in 
