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 phEenomena of absorption which 

 he regards as contrary to the theory of undulations. Among other re- 

 marks he makes the following: "That the undulatoiy theory is defective 

 as a j9%««caZ representation of the phaenomena 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 

 phaenomena of absorption. 



Airyf, in his remarks upon this paper of Brewsttr, has certainly fully 

 acknowledged that the undulatory theorj^ had hitherto given no expla- 

 nation of the phaenomena 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 tliis, 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 phaenomena of light, we must also on the 

 other hand presuppose in light a property through which its phaenomena 

 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 phaenomenon 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. 



Wlicn 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 absolutions (nearly one hundred) do not proceed from one and the 

 same cause, I was convinced that the whole was a phaenomenon of inter- 



• rhil. Mag. and Annals, vol. ii. p. 360. f Ibid. p. 419. 



