Theory of Molecular Action according to Newton's Law, 265 



what his argument it based. I can only conceive it to be the 

 assumption that the equation 



can in no case render y dependent on A. That Mr. Earn- 

 shaw admits it does not in vacuo, is evident from the fact that 

 he believes the equations he has deduced to be correct in that 

 case. He says, Phil. Mag., May, p. 373, " these, then, are the 

 equations of transmission of common light through any transpa- 

 rent medium whatever." If I am right in my conjecture, then, 

 I reply that Mr. Earnshaw is not at liberty to base so sweep- 

 ing an argument as he brings forward on any assumption 

 whatever, much less on one so little likely to be correct. I re- 

 peat, that I am unwilling to suppose that Mr. Earnshaw has 

 made use of any false reasoning, but I am convinced that any 

 one who shall peruse his paper will agree with me in affirming, 

 that with so few words devoted to explaining the influence of 

 the particles of matter it is utterly impossible for any one to 

 know what Mr. Earnshaw does mean. I am the more anxious 

 to express this fully, that I may not be accused of misinter- 

 preting the argument, and I trust it will have the effect of 

 eliciting a more full and satisfactory statement. 



On the next remark of Mr. Earnshaw I shall not dwell. It 

 has reference to the promised proof by Mr. O'Brien, that "the 

 hypothesis of finite intervals cannot be correct," and to the 

 adoption of the hypothesis of the direct action of the particles 

 of matter. I shall only observe, that so far as I can see, the 

 application of this hypothesis is insufficient, unless it be ad- 

 mitted that the particles of " matter are compound, consisting 

 of many different atoms," all of which vibrate along with the 

 particles of aether. If you allow the same assumptions to the 

 finite-interval theory, it will account for the same facts by a 

 formula very much of the same kind. It is by this means that 

 I accounted for dispersion in my 'Theory of Heat,' p. 152. 

 The equations of motion of two sets of vibrating particles were 

 first obtained by me in the Transactions of the Cambridge 

 Philosophical Society, p. 237 et seq. 



The next matter to which I will direct attention has more 

 pointed reference to myself. Mr. Earnshaw, in a paper printed 

 in the Philosophical Magazine for April, points out the pro- 

 cess which I had adopted in my first Memoir on Dispersion, 

 and adds, " the remaining four lines are used as a test of the 

 truth of the undulatory theory^' (P« 308). Where, and by 

 whom, he does not state. For my own part, I disclaim any 

 such unphilosophical opinion. What I hold is this : " that 



