104 PROCEEDINGS OF SECTION A. 
rotates the plane of polarisation of light. Then came Clerk- 
Maxwell, with his electromagnetic theory of light, and his 
“Treatise on Magnetism and Electricity”; and, finally, 
Hertz taught us how to produce electric waves, and to 
examine their properties experimentally. 
Now, we must not allow ourselves to be blinded by the 
brilliant success of the ether theory. There is nothing 
‘explanatory’ in the conception of an ether. The dis 
carded action-at-a-distance theories of W. Weber, Gauss, 
Riemann, J. and C. Neumann. and others, would have been 
‘just as good as the ether theories if only they had grouped 
together as many facts. There is little or nothing in the 
a priori objection to action at a distance; it is just as in- 
telligible (and unintelligible) as action in contact. It re- 
quires some courage to say this with Newton’s words in 
one’s ears; but we must guard against the bondage of 
authority in matters of science, whatever we think of it in 
other domains.. How far the idea of an ether is from 
“explaining” anything is also well illustrated by consider- 
ing the various modifications of ethereal structure that have 
been proposed. The story of these modifications shows very 
clearly that the one end of the ether theory is ‘to string 
together as many facts as possible. Green (in 1838) was 
the first to treat the subject on modern lines and in a 
thorough-going fashion. He was not content with mere 
descriptive theories, but tried to bring the motion of the 
ether under the sway of older dynamical “laws.’”’ Naturally, 
he began with the hypothesis that the ether is hke an 
ordinary elastic solid, capable of resisting compression and 
distortion. He found it possible to express everything in 
terms of three ethereal constants—the density, the rigidity, 
and the resistance to compression. It turns-out that in 
such a medium any disturbance will give rise to waves of 
two perfectly distinct types, one involving change of volume 
without rotation, and the other rotation and distortion with- 
out change of volume. As nothing was known in optical 
phenomena corresponding to the condensational wave, the 
first serious difficulty was to endow the ether with qualities 
that would free it from this wave. Green attempted to 
solve the problem by assuming the resistance to compression 
to be infinitely large compared with the rigidity. How- 
ever, when he came to extend his theory to crystalline media 
he was landed in serious trouble. Rankine, and subse- 
quently Lord Rayleigh, came to the assistance of the theory, 
the latter suggesting a theory of the interaction of ether and 
matter. The results, however, were not satisfactory ; in par- 
