544 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 
is to be added, and the effect, if any, observed. It is usually advantageous to cover 
the slit with a blue glass, or similar absorbing medium ; but sometimes eflfects take 
place in the bright part of the spectrum, which is intercepted by such a medium. 
When false dispersion abounds, it is well to look down on the fluid through a 
Nicol’s prism, so as to stop all light which is polarized in the plane of reflexion. 
Negative results with reference to a mutual action of the rays incident on 
sensitive solutions. 
215. The antagonistic effects of the more and less refrangible rays, which have 
been observed in certain phenomena, induced me to try whether anything of the 
kind could be perceived in the case of internal dispersion. The following arrange- 
ment was adopted for putting this question to the test of experiment. 
A tumbler was filled with a very dilute solution of sulphate of quinine, and placed 
in a pure spectrmn. As usual, the illuminated portion of the fluid consisted of two 
distinct parts, one the blue beam of truly dispersed light, corresponding to the 
highly refrangible rays, the other the beam reflected from motes, exhibiting the usual 
prismatic colours, and corresponding to the brighter of the visible rays. The fluid 
was nearly free from motes, so that the first beam was by far the brighter of the two ; 
and the second beam, without being bright enough at all to interfere with the ob- 
servation, was useful as serving to point out where the red, yellow, &c. rays lay. A 
flat prism, having an angle of about 130°, was then held in front of the vessel, with 
its edge vertical, and situated in the more refrangible part of the visible rays. The 
rays forming the two beams were thus bent in opposite directions, and the beams 
made to cross each other within the fluid ; and by turning the prism a little in both 
directions in azimuth, that is, round an axis parallel to the incident rays, it was easy 
to make sure that the beams did actually cross. But not the slightest perceptible 
difference in the blue beam was made by the passage of the red and other lowly 
refrangible rays across it. 
216. Certain theoretical views having led me to regard it as doubtful whether the 
intensity of light internally dispersed was proportional to the intensity of the incident 
rays, other circumstances being the same, I was induced to try the following experi- 
ment. 
The sun’s light was reflected horizontally through a large lens, which was covered 
by a screen containing two moderately large round holes, situated in the same hori- 
zontal plane, and a good distance apart. The beams coming through the two holes 
converged of course towards the focus of the lens, and at the same time contracted in 
width, and became brighter from the concentration of the light. For our present 
purpose, they may be regarded as cylindrical beams converging towards the focus of 
the lens. When they had approached each other sufficiently, they were transmitted 
through a blue ammoniacal solution of copper, contained in a vessel with parallel 
sides. The object of this was of course to absorb all the bright visible rays, which 
