382         ON  THE  CHANGE  OF  EEFRANGIBILITY  OF   UOHT.
copper, contained in a vessel with parallel sides. The object of this was of course to absorb all the bright visible rays, which 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.
Effect of an electric spark.    Nature nf vV.s* phtt
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 li^ht of a spirit-lamp, nothing was seen but a pale gleam of light .extending a good way into the fluid, and not only not con fined 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 eleetri-flying machine, which passed at no great distance over the surface.