PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 547 
investigate more fully the nature of these rays. Enough has, I think, been adduced 
to show that they are merely rays which there is no reason to suppose are physically 
different from those of light, but quite the contrary, and which are of very high 
refrangibility, and are therefore invisible, since they fall far beyond the limits of 
refrangibility within which the retina is affected. Indeed, it seems very likely that 
the highly refrangible rays never reach the retina, but are absorbed by the coats of 
the 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 refrangibility 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 teem phosphorescence has been applied to several different phenomena, 
I must here explain that I mean the spontaneous exhibition of a soft light, inde- 
pendently 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 froin 
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 phosphores- 
cence of Canton’s phosphorus, sulphuret of barium, and other phosphor!, is pro- 
duced, are those of high refrangibility, 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 inci- 
dent 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 Can- 
ton’s phosphorus is excited by the rays from incandescent lime, the active rays belong 
to the red extremity of the spectrurn-l-. If this result be confirmed, it follows that 
the most striking law relating to internal dispersion is not obeyed in the case of 
phosphorescence. 
In the same paper Professor Draper remarks, “Some time ago I determined the 
refrangibility of the rays of an electric spark which excite phosphorescence in sul- 
phu ret of lime ; they are found at the violet extremity of the spectrum.” In what 
way Professor Draper determined the refrangibility of rays with respect to which 
glass is so opake, he does not give the least hint. Being perfectly in the dark as to 
the evidence on which the conclusion is based, I cannot accept it in contradiction to 
* See note K. f Philosophical Magazine, vol. xxvii. (Dec. 1845) p. 436. 
