374« Prof. Kelland on the explanation of Dispersion. 



subject, from the feeling that I could add little to the impetus 

 which research had acquired in the able hands of Professor 

 Powell and others, and from the firmest reliance on their zeal 

 and desire for the advancement, not of their own views, but 

 of science itself. The appearance, however, of a new writer 

 in the field, whilst I hail it \^th real pleasure, compels me to 

 take my part, if not in what is yet to be done, at least in 

 keeping alive a due attention to what at present has been ef- 

 fected. I confess, too, that I am further influenced to the 

 present step, by the words with which your correspondent 

 closes his paper: — " I believe also that the result of this se- 

 cond approximation may be made use of to prove that the 

 hypothesis of finite intervals cannot be correct." I shall not, 

 however, digress from my path to anticipate what may be 

 the weight of the forthcoming objection to the hypothesis of 

 finite intervals, but shall confine myself strictly to the follow, 

 ing points: — 



1. To state what is the precise hypothesis I have myself 

 admitted, and refer to the conclusions based on it. ' 



2. To examine what is Mr. O'Brien's hypothesis ; and 



3. To compare his results with those previously arrived at. 

 1. In my memoir on the 'Dispersion of Light' (Trans. 



Camb. Phil. Soc, vol. vi. p. 157), the conclusions and calcu- 

 lations are based on the following suppositions, — 1st, that the 

 distance between the particles of aether is sufficiently large, com- 

 pared with tlieir difference of motion, to allow the square of 

 the latter quantity to be neglected in comparison with that of 

 the former ; 2ndly, that the same distance bears an appreciable 

 ratio to the length of a wave ; and Srdly, that the medium is 

 one of perfect symmetry. These hypotheses are conformable 

 with the views of Cauchy and others, and may be considered 

 as the simplest form of the theory of finite intervals. From 

 a conviction of the utter impossibility of an arrangement of 

 perfect symmetry, I was led to adopt, subsequently, the notion 

 of a disposition of particles similar to that deduced by M. 

 Mossotti, which is shown in Mr. O'Brien's figure 4. (See 

 Scientific Memoirs, vol. i. p. 448.) It is true I did not suc- 

 ceed in proving that the conditions resulting from such an 

 arrangement are the same as those which depend on the 

 supposition of perfect symmetry. Mr. O'Brien proposes to 

 do this, and if he succeeds, it will, I am sure, be an im- 

 portant step in our theoretical investigations. In my first 

 essay on Dispersion, the presence of material particles was 

 taken no account of. The exhibition of the results modified 

 on this account, was reserved to a subsequent memoir, to 

 which I shall refer in the sequel. It will be sufficient for my 



