PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF EIGHT. 545 
would not only be useless for exciting the solution which it was meant to try, but 
would materially hinder the observation by the glare which they would produce. 
The beams were then admitted into a vessel containing a decoction of the bark of 
the horse-chestnut, greatly diluted with water. In passing through the fluid they 
produced two blue beams of truly dispersed light, which converged towards a point a 
little way outside the vessel. A flat prism, with an angle of about 150°, was then 
held in front of the vessel, with its edge vertical, and situated between the incident 
beams. The blue beams of dispersed light were thus made to cross within the fluid ; 
and by moving the prism in azimuth, it was easy to make one beam either fall above 
the other, cross it, or fall below it. Now on looking down from above with one eye 
only, and moving the prism backwards and forwards in azimuth, I could not perceive 
the slightest difference of illumination, according as the blue beams actually crossed 
each other, or were merely seen projected one on the other. In this experiment, then, 
it appeared that one beam of incident rays produced as much additional dispersed 
light in a portion of fluid already excited by the other beam, as it was capable of 
producing in a similar portion of fluid not otherwise excited. 
Eff'ect of an electric spark. Nature of its phosphor ogmic rays. 
217- For the use of the apparatus with which the following experiments were 
made, I am indebted to the kindness of Professor Gumming. 
An electric spark produces an internal dispersion of light in a very striking manner 
in the case of an extremely dilute solution of sulphate of quinine. Having prepared 
a solution so weak, that when it was examined by superficial projection by the light 
of a spirit-lamp, nothing was seen but a pale gleam of light extending a good way 
into the fluid, and not only not confined to the surface, but not even showing any 
particular concentration in the neighbourhood of the surface, I placed it so as to be 
illuminated by the sparks from the prime conductor of an electrifying machine, which 
passed at no great distance over the surface. A very marked internal dispersion was 
produced, but the nature of the effect depended in a good measure on the character 
of the spark. A feeble branched spark, giving but little light, and making little 
noise, produced an illumination extending to a considerable depth, and very much 
stronger than that occasioned in the same solution by the flame of a spirit-lamp. 
The rays by which this was produced passed in a great measure through a plate of 
glass interposed between the spark and the surface of the fluid. But a bright linear 
spark, making a sharp crack, produced an illumination almost confined to an ex- 
cessively thin stratum adjacent to the surface of the fluid ; apd the rays by which this 
was produced were cut off by glass, though transmitted through quartz. The same 
was the case with the discharge from a Leyden jar, which produced a bright light 
almost confined to the surface*. 
218. The opacity of a solution of sulphate of quinine appears to increase regularly 
* See note J. 
4 A 2 
