264; Prof. Kelland's Reply to some Objections against the 



that has so often been started ; it in fact goes to the foundation 

 of the Jinite-interval theory. If that theory be supposed to 

 consist in the hypothesis, that the vibrations of the particles 

 of aether within a medium are unaffected by the presence of 

 the particles of matter in any shape, I shall not undertake to 

 be its advocate. I will simply refer to M. Cauchy's Memoir 

 (Prague), p. 188. But it is evident that Mr. Earnshaw 

 admits into that theory the indirect action of the particles of 

 matter ; for he says, " I have not taken account of the direct 

 action of matter upon the aether ; but as my results are inde- 

 pendent of arrangement, it is obvious that the indirect effect 

 of matter is included in them. Consequently the indirect 

 effect of matter never can assist us in accounting either for the 

 transversality of vibrations or for dispersion" (p. 48). lam 

 obliged to ask Mr. Earnshaw what he conceives to be the di- 

 rect effect of matter. The phrase was, I think, originated by 

 myself, and was meant to express the attractions or repulsions 

 of the quiescent particles of matter on those of aether. If this 

 be the sense in which Mr. Earnshaw uses the phrase, then I 

 must understand from the above quotation that he has not es- 

 timated the direct action of the particles of matter, simply 

 because he has assumed that those particles vibrate, or rather 

 perhaps, because he has assumed that they vibrate respectively 

 in precisely the same manner as the particles of aether would 

 do if they filled the same place. If this be the case, indeed, 

 whatever Mr. Earnshaw assumes, the expressions for the ve- 

 locity of transmission must contain a term due to the action of 

 the particles of matter. Let us even take the extreme case of 

 supposing that these particles are at rest, and that their attrac- 

 tions or repulsions produce no effect : still is there an indirect 

 effect due to them, which although not easily calculated, is 

 clearly of the utmost importance. I allude to the effect due 

 to the want qf action of particles of aether in the portions of 

 space occupied by the material particles. Neither this, nor 

 the pressure of the particles of matter on the adjacent particles 

 of aether tending to stop their motion, does Mr. Earnshaw say 

 one word about; and yet he asserts " that the indirect effect 

 of matter is included in his equations." How is it included ? 

 If it be replied, that the equations in p. 47 are supposed to 

 contain terms dependent on the particles of matter, then is it 

 evident that Mr. Earnshaw's argument is an antithesis to his 

 premises; the latter being the expressions for the velocity of 

 transmission in vacuo and in a refracting medium are different 

 in form, the former, therefore the velocities themselves must 

 have the same form. Now as I am not willing to accuse Mr. 

 Earnshaw of any such reasoning, I am anxious to imagine on 



