Mr. N. Campbell on the JEther. 187 



of dynamics are assumed to be true. Now those propositions 

 state that the mass of a body is independent of its state of 

 motion. When it is deduced from the equations of electro- 

 magnetism that the mass of a charged body varies with its 

 motion, the propositions of dynamics are denied to be true, 

 and, accordingly, the term " Absolute Motion " is deprived of 

 all significance. It is logically impossible to assert at the 

 same time (1) that axes fixed in the aether are axes of which 

 the Absolute Velocity is nil, and (2) that the mass of a body 

 increases with its velocity relative to these axes. If one of 

 the two propositions is taken to be true, the other becomes, 

 not false, but meaningless. 



We must assume, therefore, that adherents to the aether 

 believe that " velocity relative to the aether " is neither 

 velocity measured in the ordinary way, or Absolute Velocity. 

 And since these two meanings of " velocity " are the only 

 two employed in physics outside electromagnetism, we must 

 conclude that the velocity of electromagnetism is a new con- 

 cept and is defined by the first proposition in which it occurs. 

 Let us investigate the consequences of this conclusion. 



§ 9. There are two classes of well-known observations 

 which lead thus to a determination of the velocity of some 

 body relative to the aether. The first of these, and the most 

 direct, is represented by Rowland's experiment on the mag- 

 netic effect of moving charoes. Rowland showed that, if a 

 charge e was moving with a velocity u relative to a system of 

 observing magnets, then the charge was equivalent to a 

 current element eu. Therefore, and this is the only deduc- 

 tion possible, the velocity of the charge relative to the aether 

 is its velocity relative to the observing system of magnets. 



The second series of observations concern aberration and 

 the experiment of Michelson and Morley. It can be deduced 

 from the fundamental propositions of electromagnetism that, 

 if the velocity of an observer relative to the aether changes 

 by an amount ?(, then the apparent direction of a ray of 

 light seen by the observer is changed through an angle 



v , where 6 is the angle between the direction of the 



ray and the direction of a. Observations on stars show that 

 it is the velocity of the earth in its orbit round the sun and 6 

 the angle between that velocity and the direction of the star. 

 On the other hand, observations made on terrestrial sources 

 show that u is zero. Accordingly we have to conclude, and 

 this again is the only conclusion possible, that the velocity 

 of the observer relative to the aether is the velocity of the 

 earth in its orbit when stars are considered, and is zero when 



