296 Maxwell. 



in MacCullagh's theory the difference between the contiguous 

 media is represented by a difference of their elastic constants, 

 so in the electromagnetic theory it may be represented by a 

 difference in their specific inductive capacities. From a letter 

 which Maxwell wrote to Stokes in 1864, and which has been 

 preserved,* it appears that the problem of reflexion and refrac- 

 tion was engaging Maxwell's attention at the time when he was 

 preparing his Eoyal Society memoir on the electromagnetic 

 field; but he was not able to satisfy himself regarding the 

 conditions which should be satisfied at the interface between 

 the media. He seems to have been in doubt which of the rival 

 elastic-solid theories to take as a pattern ; and it is not unlikely 

 that he was led astray by relying too much on the analogy 

 between the electric displacement and an elastic displacement. t 

 For in the elastic-solid theory all three components of the dis- 

 placement must be continuous across the interface between two 

 contiguous media ; but Maxwell found that it was impossible to 

 explain reflexion and refraction if all three components of the 

 electric displacement were supposed to be continuous across the 

 interface ; and, unwilling to give up the analogy which had 

 hitherto guided him aright, yet unable to disprove^ the Greenian 

 conditions at bounding surfaces, he seems to have laid aside the 

 problem until some new light should dawn upon it. 



This was not the only difficulty which beset the electro- 

 magnetic theory. The theoretical conclusion, that the specific 

 inductive capacity of a medium should be equal to the square of 

 its refractive index with respect to waves of long period, was 

 not as yet substantiated by experiment; and the theory of 

 displacement-currents, on which everything else depended, was 



* Stokes's Scientific Correspondence, ii, pp. 25, 26. 



t It must be remembered tbat Maxwell pictured tbe electric displacement as a 

 real displacement of a medium. "My theory of electrical forces," he \vrote, " is 

 that they are called into play in insulating media by slight electric displacements, 

 which put certain small portions of the medium into a state of distortion, which, 

 being resisted by the elasticity of the medium, produces an electromotive force." 

 Campbell and Garnett's Life of Maxwell, p. 244. 



| The letter to Stokes already mentioned appears to indicate that Maxwell for 

 a time doubted the correctness of Green's conditions. 



