340 SCIENCE PROGRESS 



Can the issue be raised in a simple concrete fashion and 

 the distinction between the two clearly formulated in a single 

 instance ? 



Yes : consider the radiation from a uniformly moving body 

 through uniform space, all receivers and modes of detecting the 

 light moving with it. On a projectile theory and likewise 

 on the principle of relativity, successive wave-fronts — if the 

 name be still permissible for disturbances originating together 

 and still in the same phase— will be and will remain concentric 

 spheres. For on the corpuscular view those emanating in 

 the van will have the body's own speed super-added to their 

 ejection velocity, whilst those flying to the rear will fly with 

 less speed, so that their distances from the source of those 

 simultaneously ejected will remain in every direction the same. 



On the relativity view the question will not arise, for the 

 supposed motion through mere emptiness is absolute motion 

 and therefore undetectable or meaningless. The unit of time 

 may be defined as the time taken for light to travel unit 

 distance from the source in any direction ; there is no way 

 of detecting such motion of a relatively stationary system of 

 bodies as has been supposed. Pragmatically it is non-existent. 



On the ether theory, however, occurrences will be quite 

 different. The waves are now waves in a stationary medium, 

 having a definite velocity of propagation depending on the 

 properties of that medium and on nothing else, not at all 

 dependent on what the source may be doing, not in the 

 least accelerated or retarded by any motion of the source. The 

 wave-fronts are like similar swimmers setting out in regular 

 succession, in all directions, from a slowly moving ship in 

 calm water ; they will be closer together in the van than 

 in the rear but all their speeds will be identical. The corre- 

 sponding set of swimmers will remain on circles but they 

 will not be concentric circles, the ship will be steadily moving 

 away from each centre of emanation. So also the waves 

 relatively to the source will be subject to uniform drift back- 

 wards; their section will be a system of coaxial circles. 



Yes, opponents will say, we know well enough that that 

 is a consequence of the ether theory but how do you prove 

 that this occurs, how can you verify the conclusion that 

 things really happen so for a source fixed to the earth ? 

 How can you, travelling with the earth, employ any instru- 



