ON  THE  CHANGE  OF  REFRANGIBILITY OF LIGHT.         385
eye* Hence the phenomena relating to the phosphorescence produced by an electric discharge afford no countenance to the supposition that it is possible to divide rays of a given refrangi-bility into phosphorogenic, chemical, luminous, &c. Of course the most unexceptionable mode of determining the refrangibility of the phosphorogenic rays would be by actual prismatic decomposition, but this would require the employment of a quartz train.
Points of resemblance and contrast between internal dispersion and phosphorescence.
221. As the term phosphorescence has been applied to several different phenomena, I must here explain that I mean the spontaneous exhibition of a soft light, independently of chemical changes, which some substances exhibit for a time after having been exposed to the sun's rays, or to an electric discharge, or to light from some other sources.
In many respects the two phenomena have a strong resemblance. Thus, the general features of internal dispersion cannot be better conceived than by regarding the sensitive medium as self-luminous while under the excitement of the active rays. Again, it is well known that the rays of the solar spectrum by which the phosphorescence of Canton's phosphorus, sulphuret of barium, and other phosphori, is produced, are those of high refrung'ibility, as well as the invisible rays beyond; and these are precisely the rays which in the great majority of cases are most efficient in producing internal dispersion. I do not however know how far it may be true that when phosphorescence is excited by homogeneous light the refrangibility of the incident light is a superior limit to the refrangibilities of the component parts of the light emitted. Indeed, according to Professor Draper, when the phosphorescence of Canton's phosphorus is excited by the rays from incandescent lime, the active rays belong to the red extremity of the spectrumf. If this result be confirmed}, it follows that the
*  fcjeo note K.
t Philosophical Mayazine, Vol. xxvn. (Dec. 1845) p. 436.
$ [Early in 1853 I wan engaged along with Faraday in preparing in the laboratory of the Koyal Inntitution some experiments to show at an evening lecture on the subject of this paper. I expressed to him a wish to repeat Draper's experiment,
S.  III.                                                                                        ^25