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 Rayleigh, 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 
