282 BELL SYSTEM TECHNICAL JOURNAL 



radiation may not be compared with the wave-motion which is sound; 

 but a wide range of comparisons still remains open. 



Of course, very many have proposed images and models for "the 

 thing of which the vibrations are light", and many have belie\-ed 

 with an unshakable faith in the reality of their models. The fad 

 that light-waves may be compared, detail by detail, with transverse 

 vibrations in an elastic solid, led some to fill universal space with a 

 solid elastic medium to which they gave the sonorous name of "lum- 

 iniferous aether". It is not many years since men of science used to 

 amaze the laity with the remarkable conception of a solid substance, 

 millions of times more rigid than steel and billions of times rarer 

 than air, through which men and planets serenely pass as if it were 

 not there. Even now one finds this doctrine occasionally set forth.' 



In that image of the elastic solid, the propagation of light was 

 conceived to occur because, when one particle of the solid is drawn 

 aside from its norma! place, it pulls the next one aside, that one the 

 next one to it, and so on indefinitely. Meanwhile, each particle 

 which is drawn aside exerts a restoring force upon the particle of 

 which the displacement preceded and caused its own. Set one of the 

 particles into vibration, and the others enter consecutively into 

 vibration. Maintain the first particle in regular oscillation, and 

 each of the others oscillates regularly, with a phase which changes 

 from one to the next; a wave-train travels across the medium. One 

 particle influences the next, because of the attraction between them. 

 But in the great and magnificent theory of light which Maxwell 

 erected upon the base of Faraday's experiments, the propagation 

 was explained in an altogether different manner. V^ary the magnetic 

 field across a loop of wire in a periodic manner, and you obtain a 

 periodic electric force around the loop, as is known to ever\'one who has 

 dabbled in electricity. \'ary the electric field periodically, and you 

 obtain a periodic magnetic field — this a fact not by any means so 

 well known as the other, one which it was Maxwell's distinction to 

 have anticipated, and which was verified after the event. In a 

 traveling train of light-waves the electric field and the magnetic 

 field stimulate one another alternately and reciprocalh', and for this 

 reason the wa\'e-train tra\els. Since the periodic electric field may 

 point in any one of the infinity of directions in the plane of the wave- 

 front, the wa\e-motion possesses all the freedom and variability of 



' Apparently the image of the elastic solid was never quite perfected; one recalls 

 the question as to whether its vibrations were in or normal to the plane of polarization 

 of the light, which required one answer in order to agree with the phenomena of 

 reflection, and another in order to agree with those of double refraction. ProbabK 

 a modus t'ivendi could have been arranged if the whole idea had not been superseded. 



