THE LUMINIFEROUS ^THER. 117 



simultaneously, tooku^) Fresuel's view of the coustitutiou of the iether 

 aud applied it to exi)lain the laws of double refraction. In their theory 

 the conclusions arrived at were rigorously derived from the premises; 

 but the results did not altogether agree with observation; that is to 

 say, although they could by the adoption of certain sui)i)ositions be 

 forced into a near accordance with the observed laws of double refrac- 

 tion, yet they pointed out the necessity of the existence of other phe- 

 nomena which were belied by observation. Our own countryman Green 

 was the first to deduce Fresnel's laws from a rigorous dynamical theory; 

 although nearly simultaneously, MacCullagh arrived at a theory in 

 some respects similar, though on the whole I think less satisfactory. 



Still all these theories followed j)retty closely the analogy of ponder- 

 able matter; and at least in the first three mentioned the iiether was 

 even imagined to consist of discrete molecules, acting on one another, 

 like the bodies of the solar system regarded as points, by forces in the 

 direction of the joining line, and varying as some function of the dis- 

 tance. I have already quoted the very strong language in which New- 

 ton rejected the idea of the heavenly bodies acting on one another 

 across intervening si^aces which were absolutely void. But the concep- 

 tion has nothing to do with the magnitude of the intervening spaces; 

 and the conception of action at a distance across an intervening space 

 which is absolutely void, is not a bit easier when the space in question 

 is merely that separating two adjacent molecules, when the a?ther is 

 thought of as consisting of discrete molecules, than it is when the space 

 is that separating two bodies of the solar system, though in this latter 

 case it may amount to many millions of miles. If the tether be in some 

 unknown manner the link of connection whereby two heavenly bodies 

 are enabled to exert on one another the attraction of gravitation, then 

 according to the hypothetical constitution of the aether that we have 

 been considering, we seem comftelled to invent an lether of the second 

 order, so to speak, to form a link of connection between two separate 

 molecules of the luminiferous a*ther. But since the nature of the a'ther 

 is so very different as it must be from that of i^onderable matter, it may 

 be that the true theory must proceed ui^on lines in which our previous 

 conceptions derived from the study of ponderable matter are in great 

 measure departed from. 



If we think of the tether as a sort of gigantic jelly, we can hardly 

 imagine but that it would more or less resist the passage of the heav- 

 enly bodies — the planets for instance — through it. Yet there appears 

 to be no certain indication of any such resistance. It has been 

 observed indeed in the case of Encke's comet, that at successive revo- 

 lutions the comet returned to its perihelion a little before the calcu- 

 lated time. This would be accounted for by the supposition that it 

 experienced a certain amount of resistance from the aether. Although 

 at first sight we might be disposed to say that such a resistance would 

 retard perihelion passage, yet the fact that it would accelerate it 



