xlvi REPORT—1852. 
those who, as in the case of these private observatories, have already largely 
contributed from their own resources. 
The Mathematical and Physical Theories of Zight have afforded subjects 
for many interesting and profitable discussions in Section A, and have usually 
had one day in the six specially allotted to them. Those discussions will 
derive a more than usual interest at this meeting from the remarkable dis- 
covery recently made by Prof. Stokes, that under certain circumstances a 
change is effected in the refrangibility of light, and from the advantage we 
possess in having amongst us on this occasion the eminent mathematician 
and physicist by whom this most important contribution to the science of 
physical optics has been made. His researches took their origin from an 
unexplained phenomenon discovered by Sir John Herschel and communicated 
by him to the Royal Society in 1845. A solution of sulphate of quinine exa- 
mined by transmitted light, and held between the eye and the light, or 
between the eye and a white object, appears almost as transparent and colour- 
Jess as water; but when viewed in certain aspects and under certain 
incidences of light, exhibits an extremely vivid and beautiful celestial blue 
colour. This colour was shown by Sir John Herschel to result from the 
action of the strata which the light first penetrates on entering the liquid; 
and the dispersion of light producing it was named by him epipolic disper- 
sion, from the circumstance that it takes place near the surface by which the 
light enters. A beam of light having passed through the solution was to all 
appearance the same as before its entrance ; nevertheless it was found to have 
undergone some mysterious modification, for an epipolised beam of light, 
meaning thereby a beam which had once been transmitted through a quini- 
ferous solution, and had experienced its dispersive action, is incapable of fur- 
ther epipolic dispersion. In speculating upon the possible nature of epi- 
polised light, Prof. Stokes was led to conclude that it could only be light 
which had been deprived of certain invisible rays which in the process of 
dispersion had changed their refrangibility and had thereby become visible. 
The truth of this supposition, novel and surprising as it at first appeared, has 
been confirmed by a series of simple and perfectly decisive experiments ; 
showing that it is in fact the chemical rays of the spectrum more refrangible 
than the violet, and invisible in themselves, which produce the blue superficial 
light in the quiniferous solution. Professor Stokes has traced this principle 
through a great range of analogous phenomena, including those noticed by 
Sir David Brewster in his papers on “Internal Dispersion,” and has distin- 
_ guished between: “ cases of false internal dispersion” or “ opalescence,” in 
which the luminous rays are simply reflected from fine particles held in me- 
chanical solution in the medium, and those of “true internal dispersion,” or 
“fluorescence,” as it is termed by Mr. Stokes. By suitable methods of ob- 
servation the change of refrangibility was detected, as produced not only by 
transparent fluids and solids, but also by opake substances; and the class of 

