PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 469 
more decisive by using a greater thickness, as well as to render the observation strictly 
differential, I placed a tumbler filled with water behind the slit, the blue glass before 
it, and then viewed the slit through the prism. 1 saw as far as usual into the violet. 
The water was then poured out and replaced by the solution of sulphate of quinine, 
which, when viewed by transmitted light, appeared as transparent as the water which 
it had replaced. When the tumbler was now placed behind the slit, the blue beam 
of dispersed light was observed to extend quite across it, a distance of about three 
inches, and would evidently have gone much further. On viewing the slit through 
the prism, the spectrum was found to be cut off about half-way between the fixed 
lines G and H. The termination was pretty definite, which indicates that, at least 
for that part of the spectrum, the absorbing energy of the fluid rapidly increased with 
the refrangibility of the light ; there was, however, an evident diminution of intensity 
produced by the fluid, extending from the termination of the spectrum to near G. 
12. There could no longer be any doubt, either as to the fact of a change of re- 
frangibility, or as to the explanation thereby of the remarkable phenomenon exhi- 
bited by sulphate of quinine. Epipolized light is merely light which has been purged 
of the invisible, or at most feebly illuminating rays more refrangible than the violet ; 
and the term itself, which in fact was only adopted provisionally by Sir John 
Herschel, and which has now served its purpose, may henceforth be discarded, 
especially as it is calculated to convey a false impression respecting the cause of the 
phenomenon. It remained to examine other instances of internal dispersion, of 
w'hich, according to Sir David Brewster’s observations, the dispersion produced by 
sulphate of quinine is only a particular case ; to endeavour to make out the laws 
according to which a change of refrangibility takes place ; and, if possible, to 
account for these laws on mechanical principles. 
13. In giving an account of my further experiments, I think it best to describe in 
detail the phenomena observed in some of the more remarkable instances of internal 
dispersion before attempting to draw any general conclusions. It will save repetition 
to explain in the first instance the methods of observation employed, which on the 
whole may very fairly be divided into four, though occasionally it was convenient to 
employ intermediate methods, or a combination of two of them. Of course I fre- 
quently availed myself of Sir David Brewster’s method of observation, in which the 
effect of the incident light is studied as a whole ; but the methods here referred to 
relate to an investigation of the separate offices of the portions of light of different 
degrees of refrangibility which are found in the incident beam. As my researches 
proceeded, new methods of observation suggested themselves, but these will be 
described in their place. 
Methods of Observation employed. 
First Method. — The sun’s light was reflected horizontally through a small lens, 
which was fixed in a hole in a vertical board. The cone of emergent rays was 
