358 Mr. S. Tolver Preston on Method 



maticians of the beginning of the nineteenth century has been 

 expended." 



Faraday, Davy, and Rumford are known to have opposed 

 the notion of " action at a distance." It is thus refreshing to 

 see that the greatest leaders of physical science, those who 

 have added most to knowledge and contributed most to real 

 progress, have always been averse to this kind of spiritualistic 

 doctrine. 



In the Report of the British Association for 1862 is a 

 paper by Prof. Stokes, entitled " Report on Double Refrac- 

 tion"*, to which I refer here, as this paper gives an account 

 serving well to illustrate some of the different theories of the 

 aether. I would particularly beg of those who are singly inter- 

 ested in the advancement of knowledge, and who would desire 

 to take an unbiassed and unprejudiced view of the case, to give 

 a share of attention to the following points which I shall 

 develop. I need not add that I do not enter on this subject 

 lightly, having already devoted some years of study to the 

 question. First the theory of Fresnel is mentioned ; and the 

 Report states of it (p. 253) that the aether " is regarded as 

 consisting of distinct material points, symmetrically arranged, 

 and acting on one another with forces depending, for a given 

 pair, only on the distance." 



Here, it will be observed, we have the arbitrary assumption 

 about "force" (or "action at a distance"). It is stated 

 further in the Report that " Fresnel distinctly makes the sup- 

 position that the aether is incompressible, or at least is sensibly 

 so under the action of forces comparable with those with which 

 we are concerned in the propagation of light." Prof. Stokes 

 then raises the probably not unnatural question whether this 

 supposition of " incompressibility is not inconsistent with the 

 assumed constitution of the aether" (given above); and he 

 adds (page 253), " I have mentioned these points,, because 

 sometimes they are slurred over, and Fresnel's theory spoken 

 of as if it had been rigorous throughout." 



The theories of Cauchy and Neumann are then referred to. 

 It is stated that, according to their view (page 254), " the 

 aether is supposed to consist of distinct particles, regarded as 

 material points, acting on one another by forces in the line 

 joining them which vary as some function of the distances ; 

 and the arrangement of these particles is supposed to be dif- 



* My attention was directed to this Report by a reference to it by the 

 late Prof. Clerk Maxwell, in the article " Ether " in the new edition of 

 the Encyc. Brit, where it is also remarked — u The undulatory theory, in 

 the form which treats the phenomena of light as the motion of an elastic 

 solid, is still encumbered with several difficulties." 



