86 



SCIENTIFIC THOUGHT. 



optical, electrical, and magnetic phenomena, which by 

 carefully devised experiments might be verified and 

 extended. 



Through Maxwell, following on Faraday and Thomson, 

 the treatment of electric and magnetic phenomena had 

 thus entered on a similar stage to that which the 

 treatment of optical phenomena had attained half a 

 century earlier through Young and Fresnel. A kinetic 

 or mechanical view, more or less precise and definite, had 

 been propounded ; a considerable number of facts had 

 been brought into connection, into line and order ; the 

 direction which experimental research must take had 

 been indicated ; and finally a correspondence had been 

 established between two great groups of phenomena, those 

 of electricity and magnetism on the one side, those of 

 light on the other. It might have been expected that 

 Maxwell would now take the same course as that taken 

 by Fresnel about the year 1820, and perfect his views 

 by giving his theory of molecular vortices greater pre- 

 cision and definiteness i.e., by perfecting the electro- 

 magnetic model, as Fresnel and others perfected in their 

 time the system of vibrations by which they visualised 

 the processes of light. This is not the method which 

 Maxwell adopted. 1 In his later and more important 



J The progress of Maxwell's reason- 

 ing is clearly marked in the three 

 memoirs, belonging respectively to 

 the years 1855, 1861, and 1864, of 

 which the last appeared in the 

 ' Transactions ' of the Royal So- 

 ciety, and which are reprinted in 

 the first volume of the ' Collected 

 Scientific Papers.' The first memoir 

 on " Faraday's Lines of Force " ad- 

 heres strictly to the mathematical 



formulation of Faraday's concep- 

 tion, much in the spirit of Thom- 

 son's many expositions. The second, 

 on "Physical Lines of Force," fol- 

 lows Faraday in the attempt to take 

 the original symbol in real earnest 

 as a physical arrangement, and de- 

 vises, or applies for that purpose, 

 the theory of molecular vortices. 

 The third memoir, which is by far 

 the most important and original, 



