j6 Notes on some Points in the Theory of Light. 



ing the subsequent additions for which we are indebted to 

 Mr. Airy, but leading to new results, one of which establishes 

 a relation between two different classes of phenomena, and 

 is verified by the experiments of M. Biot and Mr. Airy. 

 Having, therefore, such conclusive proofs of the truth of these 

 equations, we are entitled to assume them as a standard 

 whereby to judge of any theory ; so that any mechanical 

 hypothesis which leads to results inconsistent with them may 

 be at once rejected. 



Now I assert that the mechanical hypothesis of M. Cauchy 

 contradicts these equations, and therefore contradicts all the 

 phenomena and experiments which he supposed it to repre- 

 sent. But before we proceed to the proof of this assertion, 

 it may perhaps be proper to remark, that previously to the 

 date of M. Cauchy's communication, and of my own Paper, I 

 had actually tried and rejected this identical hypothesis, and 

 had even gone so far as to reject along with it the whole of 

 M. Cauchy's views about the mechanism of light. For though, 

 in my Paper, I have said nothing of any mechanical investiga- 

 tions, yet, as a matter of course, before it was read to the Aca- 

 demy, I made every effort to connect my equations in some 

 way with mechanical principles; and it was because I had 

 failed in doing so to my own satisfaction, that I chose to 

 publish the equations without comment,* as bare geometrical 

 assumptions, and contented myself with stating orally to the 

 Academy, as I did some months after to the Physical Section 

 of the British Association in Bristolf that a mechanical account 

 of the phenomena still remained a desideratum which no attempts 

 of mine had been able to supply. I am not sure that on the first 

 occasion I stated the precise nature of these attempts, though I 



* The circumstances here related will account for what Mr. Whewell (History 

 of the Inductive Sciences, VOL. n. p. 449) calls the " obscure and oracular form " in 

 which those equations were published. Having, at that time, no good explanation 

 of them to give, I thought it better to attempt none. But in the general view which 

 I have since taken (see p. 224 of this volume), they do not offer any peculiar diffi- 

 culty. 



t See " Transactions of the Sections," p. 18. 



