PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITV OF LIGHT. 555 
manner of an isolated vibrating plate or rod, and between these extreme limits we 
may conceive various intermediate modes of vibration. Hence, without departing 
from the general supposition that the absorption of light is due to the production of 
molecular disturbances, we may conceive that the modes in which the ether commu- 
nicates its vibrations to the molecules, and the molecules in turn communicate their 
disturbances to the ether, are very various. 
I do not bring forward the idea that the absorption of light is due to the produc- 
tion of molecular disturbances as new, though possibly the communication of the 
ethereal vibrations to the molecules may hitherto have been supposed necessarily to 
imply the existence of synchronous vibrations among the molecules. The change in 
the periodic time of vibrations which takes place in the process of internal dispersion 
would hardly have been suspected, had it not been for the singular phenomenon 
which pointed it out. 
2.38. The only theory of absorption, so far as I am aware, in which an attempt is 
made to deduce its laws from a physical cause is that of the Baron Von Wrede, who 
attributes absorption to interference*. The Baron’s paper is in many respects very 
beautiful, but it has always appeared to me to be a fatal objection to his theory that 
it supposes vibrations to be annihilated. It is true that two streams of light may 
interfere and produce darkness, but then to make up for it more light is produced in 
other quarters. Light is not lost by interference, but only the illumination differently 
distributed. Were the disappearance of light in the direction of a pencil admitted 
into a medium merely a phenomenon of interference, the full quantity of light admitted 
ought to be forthcoming in side directions. Were a series of vibrations incident on 
a medium, without producing any progressive change in its state, or any disturbance 
issuing from it, it would follow that work was continually being annihilated. But 
we have reason to think that the annihilation of work is no less a physical impossi- 
bility than its creation, that is, than perpetual motion. 
List of highly sensitive substances. 
239. For the sake of any one who may wish to make experiments in this subject, 
I subjoin a list of the more remarkable of the substances which have fallen under 
my notice. It will be seen that most of these substances were suggested by the 
papers of Sir David Brewster and Sir John Herschel. 
Glass coloured by peroxide of uranium ; yellow uranite ; nitrate or acetate of the 
peroxide. Probably various other salts of the peroxide would do as well. The 
absorption bands of the salts, whether sensitive or not, of peroxide of uranium ought 
to be studied in connexion with the change of refrangibility. 
A solution of the green colouring matter of leaves in alcohol. To obtain a solu- 
tion which will keep, it is well previously to steep the leaves in boiling water. The 
alcohol should not be left permanently in contact with the leaves, unless it be wished 
* Poggendorff’s Annalen, B. xxxiii. S. 353 ; or Taylor’s Scientific Memoirs, vol. i. p. 477. 
