474 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 
which, with the stronger solution, it could not be so conveniently made out, inasmuch 
as the posterior surface of the space from which the dispersed light came almost con- 
founded itself with the anterior surface of the fluid. 
The high degree of opacity with regard to rays of great refrangibility which the 
addition of so small a proportion of sulphate of quinine is sufficient to produce in 
water is certainly very remarkable ; nevertheless it is only what I have constantly 
observed while following out these researches. 
19. In observing by the fourth method, the part of the spectrum to which the in- 
cident light belonged was determined sometimes by the colour, sometimes by means 
of the fixed lines of the spectrum. It almost always happened that there were motes 
enough suspended in the fluid to cause a portion of the dispersed beam to consist 
merely of light which had undergone ordinary reflexion. When the whole dispersed 
beam was analysed by a prism, the beam which consisted of light reflected from 
motes was separated from the rest ; it was in general easily recognised by its spark- 
ling appearance, but at any rate was known by its consisting almost wholly of light 
polarized in the plane of incidence, whereas the truly dispersed light was unpolarized. 
It consisted of course of light of definite refrangibility, the same as that of the in- 
cident light, and thus served as a fiducial line to which to refer by estimation the 
refrangibilities of the component parts of the dispersed light. Of course this part of 
the observation was possible only when the incident rays belonged to the visible part 
of the spectrum. 
On moving the lens horizontally through the colours of the spectrum, in a direction 
from the red to the violet, it was found that the dispersion was first perceptible in the 
blue. When the dispersed light was separated by a prism from the light reflected 
from motes, it was found to consist of an exceedingly small quantity of red ; further 
on some yellow began to enter into its composition ; further still, perhaps about the 
junction of the blue and indigo, the dispersed beam began to grow brighter, and 
was found on analysis to contain some green in addition to the former colours. In 
the indigo it got still brighter, and when viewed as a whole was somewhat greenish. 
Further still it became something of a pale slaty blue, and was found on analysis to 
contain some indigo, or at least highly refrangible blue. On proceeding further the 
dispersed light became first of a deeper blue and then, a little short of the fixed line 
H, whiter. At a considerable distance beyond H the dispersed light was if anything 
a shade more nearly white. 
By this method of observation the dispersion can be detected earlier in the 
spectrum than by the third method, and moreover the change in the colour of the 
dispersed light is much more easily perceived ; indeed the most striking part of this 
change takes place while the dispersed light is so very faint that it can hardly be seen 
in observing by the third method ; moreover, even in the bright part of the dispersed 
beam, it is not at all easy by the latter method to make out the change of tint corre- 
sponding to a change in the refrangibility of the incident rays, because the tint 
