478 BARON VON WREDE ON THE ABSORPTION OF LIGHT 
and has enlarged our knowledge in this as well as in the other branches 
of optics, gives (in the paper in which he describes the curious dis- 
covery that certain coloured gases possess the property of absorbing a 
countless multitude of species of light, while they freely transmit others 
lying between these) a collection of those phenomena of absorption which 
he regards as contrary to the theory of undulations. Among other re- 
marks he makes the following: “That the undulatory theory is defective 
as a physical representation of the phenomena of light, has been admitted 
by the more candid of its supporters; and this defect, insofar as it relates 
to the dispersive power of bodies, has been stated by Sir John Herschel 
as a ‘most formidable objection.’ That there are other objections to it, as 
a physical theory, I shall now proceed to show; and I shall leave it to 
the candour of the reader to determine, whether they are more or less 
formidable than that which has been stated*.” All these new objections 
of Brewster against the theory of undulations are derived from the 
phenomena of absorption. 
Airy +, in his remarks upon this paper of Brewsttr, has certainly fully 
acknowledged that the undulatory theory had hitherto given no expla- 
nation of the phenomena of absorption; but he has on the other hand 
compared the two rival theories in other respects, with so much know- 
ledge and clearness that I think it impossible after perusal of this col- 
lection of facts to hesitate for a moment which to prefer. I am however 
obliged to controvert what Airy has intimated in relation to the absorp- 
tion of light, if indeed my own view of this subject be correct. He says 
that he did not think that absorption could be considered as an essential 
part of the theory of light. “It is,” says he, “a sort of extraneous in- 
terruption, which either leaves the ordinary laws in full vigour, or whol- 
ly destroys, not the laws, but that which is the subject of the laws.” 
Granting this, I do not see how the conclusion may be drawn from it, 
that the theory of light need not include the absorption. If we pre- 
suppose in bodies certain properties by means of which they act in a 
disturbing manner on the phenomena of light, we must also on the 
other hand presuppose in light a property through which its phenomena 
would allow themselves to be disturbed by the bodies, and we must then 
necessarily explain this last property by the theory of light. I have 
pursued with attention the interesting phenomenon described by Brew- 
ster; but far from drawing the same consequence as he has done, I think 
I find in it only a complete confirmation of the theory of light. 
When I saw for the first time the spectrum of a light which had tra- 
versed iodic or bromic gas, whose regularity leaves no doubt that all 
the absorptions (nearly one hundred) do not praceed from one and the 
same cause, I was convinced that the whole was a phenomenon of inter- 
* Phil. Mag. and Annals, vol. ii. p.360. + Ibid. p. 419. 
