Closing Years of the Nineteenth Century. 447 



would have the great advantage of explaining the contraction 

 postulated by Fitz Gerald, since it would represent the con- 

 traction as actually produced by the motion. But if this 

 assumption be correct, the theory of electricity and aether is 

 without doubt the fundamental theory of Natural Philosophy ; 

 and the framework of space and time should be chosen with 

 a view chiefly to the expression of electrical phenomena. This 

 may most naturally be done by stipulating that the wave- 

 fronts of disturbances generated in free aether shall, in the 

 system of length and time adopted, be accounted spheres whose 

 centres are at the origins of disturbance and whose radii are 

 proportional to the times elapsed since their initiation. Eeferred 

 to axes of (#,y,z,) which satisfy these conditions, the fundamental 

 equations of the electric field assume the form which has been 

 taken as the basis of all our theoretical investigations. 



Imagine now a distant star which is moving with a uniform 

 velocity w or c tanh a relative to this framework (x, y, z, t). The 

 theorem of transformation shows that there exists another 

 framework (a?,, y\,z\, t^, with respect to which the star is at rest, 

 and in which moreover the condition laid down regarding the 

 wave-surface is satisfied. This framework is peculiarly fitted 

 for the representation of the phenomena which happen on the 

 star ; whose inhabitants would therefore naturally adopt it as 

 their system of space and time. Beings, on the other hand, who 

 dwell on a body which is at rest with respect to the axes 

 (x, y, z, t) would prefer to use the latter system ; and from the 

 point of view of the universe at large, either of these systems 

 is as good as the other. The equations of motion of the aether 

 are the same with respect to both sets of coordinates, and 

 therefore neither can claim to possess the only property which 

 could confer a primacy namely, an absolute relation to the 

 aether.* 



To sum up, we may say that the phenomena whose study 

 is the object of Natural Philosophy take place each at a definite 



* This was first clearly expressed by Einstein, Ann. d. Phys. xvii (1905), 

 p. 891. 



