346 Prof. Kelland's Vindication of himself against 



assertions about what I have said or denied suffice to effect it. 

 Let us take an instance or two, and I will add all that can 

 be added to such assertion by way of answer. Mr. O'Brien 

 says, "I have arrived at a result never obtained before, namely, 

 that dispersion must arise from the direct action of the par- 

 ticles of matter upon those of aether. This result is denied 

 by Professor Kelland." I answer, / never denied it. Of course 

 I object to the word must. The facts are simply these. 

 With regard to the action of the particles of matter on those 

 of aether, M. Cauchy and Mr. O'Brien adopt one hypothesis, 

 whilst I had adopted another. On theirs, the velocity of trans- 

 mission depends on the direct action of the particles of matter ; 

 on mine, it does not. That either or both may be wrong is 

 perfectly possible ; in groping after truth, we cannot reason 

 directly from data up to laws, but must work our way back 

 from assumed laws to the experimental data. Hence the value 

 of researches such as those before us. They may ultimately 

 lead to truth even at the time when least approaching to it. 

 I ought to point out that M. Cauchy regards his equation, 

 Nouveaux Exercices, vol. i. p. 98, as embodying the explana- 

 tion of dispersion by the direct action of the particles of matter 

 on those of aether. I am not aware, however, that he has 

 ever stated so in print, nor do I wish to rob Mr. O'Brien of 

 the credit of the explanation. 



But this leads me to another of Mr. O'Brien's assertions. 

 " I would ask Professor Kelland, is it possible that he thinks 

 this formula capable of accounting for dispersion independ- 

 ently of the hypothesis of finite intervals ? Is it not very evi- 

 dent, except that hypothesis be true, that kAx is extremely 

 small, &c. ? Why then has Professor Kelland produced this 

 expression as equivalent to mine ? " My best answer to this 

 will be to direct your readers to turn back (which I trust they 

 will not omit to do) and see what I have said. They will find 

 the following sentence (and I trust they will in all cases read 

 the context) : — " But that effect depends on their mutual di- 

 stances, and thusjinite intervals, not indeed of the particles 

 of aether, but of those of matter, necessarily play a conspicuous 

 part:" and again, "the real difference between the received 

 theory and that before us is this ; that the former rejects the 

 direct attraction of the particles of matter as producing no 

 effect on the time of vibration of a particle of aether," &c.&c* 

 I was perhaps hardly justified in using the word received, but 

 against this there can be no present complaint. 



Again, Mr. O'Brien says, rt I have given a simple proof of 



* See Phil. Mag., S. 3. No. 132, (vol. xx.) p. 377. 



