MATTER, SPACE, AND TIME 223 



which often proceeds with high velocity, but, as far 

 as observation goes, is entirely unimpeded. Rays 

 of light from the stars appear to reach the earth in 

 straight lines, suffering no deflection on passing 

 through the aether outside the atmosphere near the 

 earth. This result suggests that the luminiferous 

 medium is not disturbed by the movement through 

 it of the earth with a velocity of eighteen miles a 

 second — the speed with which the earth moves 

 round the sun. On the other hand, the passage 

 of light over the surface of the earth is not affected 

 by a change in direction relative to the earth's 

 total motion, the velocity of the light is the same 

 whether it is passing with or against the motion 

 of the earth. This result indicates at first sight 

 a conclusion opposed to that formerly reached, and 

 suggests that the aether is at rest relatively to the 

 surface of the earth and is dragged along with the 

 ground as it moves. This seeming discrepancy 

 led at a later date to Einstein's theory of 

 Relativity. The general dynamical problem of 

 constructing a model of the aether on ordinary 

 mechanical ideas of wave propagation never was 

 accomplished satisfactorily during the years when 

 it appeared to be perhaps possible. 



As long as the aether was invoked only to 

 explain the phenomena of light, the difficulties of 

 interpretation might well suggest doubts about the 

 fundamental hypothesis as to its existence, but 

 when Clerk Maxwell showed that it was possible 

 to explain the phenomena of the electro-magnetic 

 field by an aether having properties identical with 

 those of the luminiferous medium, the evidence 

 for both theories was strengthened very greatly. 

 Maxwell proved mathematically that the velocity 



