108 THE POPULAR SCIENCE MONTHLY 



of sound, but with a different velocity from that of transverse waves. 

 But such longitudinal waves have no place in any optical phenomenon, 

 and therefore constitute a difficulty for the theory. In order not to have 

 them it was necessary for Green to suppose the ether incompressible. 

 Thus the theory did very well for the propagation of light on free space. 

 When light passes from free space to a transparent substance, however, 

 it is partially reflected and partially refracted, travelling with a differ- 

 ent velocity in the new medium. This change of velocity could be ex- 

 plained by a difference of either density or rigidity in the two media. 

 Green chose one hypothesis, in fact the same as that of Fresnel, Neu- 

 mann and McCullagh the other. This difference gave rise to a contro- 

 versy over the direction of the vibration, as to whether it was in or 

 perpendicular to the plane of polarization, a controversy vainly sought 

 to be settled by experiment. Although reflection and refraction could 

 thus be explained, there remained a very grave difficulty. The condi- 

 tions to be satisfied at the surface between two different media are too 

 many to be satisfied by a transverse wave alone, so that had there been 

 originally only a transverse wave, it would give rise to a longitudinal 

 wave on striking the surface limiting the media. To avoid this diffi- 

 culty a mechanical theory was proposed by McCullagh, in which the 

 elasticity was not like that found in any known substance, but was 

 called into play when a portion of the medium was rotated, quite inde- 

 pendently of whether neighboring portions were rotated or not. This 

 theory gave a very satisfactory explanation of reflection and refraction, 

 but long met with opposition on account of its postulating elastic 

 properties not found in any substance. 



Probably the person who took most seriously the view of the ether 

 as having the properties of some familiar sort of matter was Lord Kel- 

 vin, who devoted a large portion of his life to the attempt to find a 

 suitable mechanical representation of the ether. In fact he stated on 

 the occasion of his jubilee that for forty years this question had not 

 been absent from his mind for a single day. Lord Kelvin frequently 

 uses the term " jelly " as typical of Green's elastic substance, and did 

 finally, by a very ingenious assumption, succeed in assimilating the 

 ether to such a substance. But in spite of all these attempts, we may 

 agree with the opinion of Lord Eayleigh, who concludes that for many 

 reasons " the elastic solid theory, valuable as a piece of purely dynam- 

 ical reasoning, and probably not without mathematical analogy to the 

 truth, can in optics be regarded only as an illustration." 



Such was the condition of affairs at the close of what I may call 

 the medieval period in optics, when, in 1864, Maxwell gave affairs an 

 entirely new turn by the presentation of his famous paper on " A Dy- 

 namical Theory of the Electromagnetic Field." In this he was guided 

 by the conjecture of Faraday that the same medium which is concerned 



