548 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY X)F LIGHT. 
in y own experiments. Perhaps, however, “at the violet extremity” may mean no- 
thing more than somewhere in the highly refracted region beyond the visible rays. 
If so, Professor Draper’s statement is in accordance with my own conclusions. 
222. When one part of a phosphorus has been excited, the phosphorescence is 
found gradually to extend itself to the neighbouring parts. In this respect a sub- 
stance which exhibits internal dispersion presents a striking contrast. The finest 
fixed lines of the spectrum are seen sharply defined, whether in a solution, or in a 
clear solid, or on a washed paper. 
223. Of course, theoretically, there ought, to a certain extent, to be a communication 
of illumination from one part of a sensitive fluid to another, on account of the light 
which is twice, three times, &c. dispersed. This however must be excessively small ; 
for the mean refrangibility of the dispersed light is usually much lower than the 
refrangibility of the active light, perhaps lower than that of any light capable of ex- 
citing the solution. However, generally some few of the dispersed rays would have a 
refrangibility sufficiently high to be dispersed again. But practically the intensity 
of the light twice dispersed in this manner would be so very small that it may safely 
be altogether disregarded. 
224. But by far the most striking point of contrast between the two phenomena, 
consists in the apparently instantaneous commencement and cessation of the illumi- 
nation, in the case of internal dispersion, when the active light is admitted and cut 
off. There is nothing to create the least suspicion of any appreciable duration in the 
effect. When internal dispersion is exhibited by means of an electric spark, it 
appears no less momentary than the illumination of a landscape by a flash of light- 
ning. I have not attempted to determine whether any appreciable duration could be 
made out by means of a revolving mirror. 
225. d'here appears to be no relation between the substances which exhibit a change 
of refrangibility and those which phosphoresce, either spontaneously, or on the appli- 
cation of heat. Thus the sulphurets of calcium and barium, on being examined for 
internal dispersion, were found to be insensible, as was also Iceland spar. The last 
substance phosphoresced strongly on the application of heat. So far as was examined, 
the minerals which did exhibit a change of refrangibility showed no special dispo- 
sition to phosphoresce. Sir David Brewster has remarked, that a specimen of 
fluor-spar which exhibited a blue light by internal dispersion, exhibited when 
heated a blue pbosphorescent light ; but this appears to have been merely a casual 
coincidence*. 
On the Cause of True Internal Dispersion, a7id of Absorption. 
226. In considering the cause of internal dispersion, we may I think at once 
discard all supposition of reflexions and refractions of the vibrations of the lumini- 
ferous ether among the ultimate molecules of bodies. It seems to be quite contrary 
* Report of the Meeting of the British Association at Newcastle in 1839, p. 11. 
