PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 557 
(5.) The phenomenon of a change of refrangibility proves to be extremely common, 
especially in the case of organic substances such as those ordinarily met with, in 
which it is almost always manifested to a greater or less degree. 
(6.) It affords peculiar facilities for the study of the invisible rays of the spectrum 
more refrangible than the violet, and of the absorbing action of media with respect 
to them. 
(/.) It furnishes a new chemical test, of a remarkably searching character, which 
seems likely to prove of great value in the separation of organic compounds. The 
test is specially remarkable for this, that it leads to the independent recognition of 
one or more sensitive substances in a mixture of various compounds, and shows to a 
great extent, before such substances have been isolated, in what menstrua they are 
soluble, and with what agents they enter into combination. Unfortunately, these 
observations for the most part require sunlight. 
(8.) The phenomena of internal dispersion oppose fresh difficulties to the supposi- 
tion of a difference of nature in luminous, chemical, and phosphorogenic rays, but are 
perfectly conformable to the supposition that the production of light, of chemical 
changes, and of phosphoric excitement, are merely different effects of the same cause. 
The phosphorogenic rays of an electric spark, which, as is already known, are inter- 
cepted by glass, appear to be nothing more than invisible rays of excessively high 
refrangibility, which there is no reason for supposing to be of a different nature from 
rays of light. 
NOTES ADDED DURING PRINTING. 
Note A. Art. 23. 
Shortly after the preceding paper was forwarded to the Royal Society, I found M. Edmond 
Becquerel’s map of the fixed lines of the chemical spectrum, which is published in the 40th 
volume of the ‘ Bibliotheque Universelle de Geneve’ (July and August 1842). I had seen in 
Moigno’s ^Repertoire d’Optique Moderne,’ that the map had been presented to the French Aca- 
demy, and naturally felt anxious to obtain it ; but not finding any further notice of it either in that 
work or in the ‘ Comptes Rendus,’ I supposed that it had not yet been published. The principal 
lines in this map I recognized at a glance. M. Becquerel’s broad band I is my /; his group of 
four lines M with the preceding band forms my group m ; his group of four lines N forms the first 
four of my group n ; his line O is my n. It is only in the last group that there can be any doubt 
as to the identification ; but I feel almost certain that M. Becquerel’s P is my o, and the next two 
lines, the last in his map, are the two between o and p. It is difficult at first to believe that the 
strong line p should have been left out, while the two faint lines between o and p are represented, 
but the difficulty is, I think, removed by considering the feeble photographic action in that part of 
