Mr. N. Campbell on the yEther. 189 



aether had been found which would meet with universal 

 approval. 



§ 11. But the approval has not been universal. M. Poincare 

 has attacked the scheme on the ground that it needs a fresh 

 assumption whenever the delicacy of our instruments is 

 increased. And it has also occurred to many people that 

 there is something very unsatisfactory in introducing into 

 the fundamental equations of a science a quantity which 

 cannot be measured experimentally, either directly or with 

 the help of those equations. 



It is probable that the future historian of physics will be 

 astounded that the vast majority of physicists should accept 

 a system of such bewildering complexity and precarious 

 validity rather than abandon ideas which seem to have their 

 sole origin in the use of the word ,; aether," and reject those 

 to which so many lines of thought point insistently. Unless 

 a perfectly arbitrary assumption is made as to the value of 

 the u velocity of the aether " relative to some observing 

 system, observation forces us to the adoption of the principle 

 of relativity — to the belief that the axes ''fixed in the aether," 

 to which Maxwell's equations must be referred, are axes 

 fixed in the charged system which is the source of the energy 

 of which the transformations are investigated. It has been 

 asserted that such ideas are really even less satisfactory than 

 those based on the conception of a single aether, because 

 " they require such a very complex structure for the aether." 

 But if we abandon the use of the word "aether" their 

 essential simplicity appears. The system in which electro- 

 magnetic energy is localized ceases to be a single body 

 independent of all material bodies ; it becomes a collection 

 of portions which are to be regarded as parts of every 

 separately moving charged body; if the charged body is in 

 uniform motion relative to the observer the portion of the 

 aether in which its energy is localized moves with the same 

 velocity relative to that observer. The principle of relativity 

 does not complicate our interpretation of electrical phenomena; 

 it simplifies it in reducing by one the number of bodies that 

 have to be taken into account. 



§ 12. It would be easy to proceed to attack in like manner 

 other confusions to which the use of the concept 4 * aether " 

 has given rise, to analyse the many and mutually inconsistent 

 attempts which have been made to estimate its density, 

 rigidity, and even atomic weight. My object is not to 

 marshal all the arguments that might be brought against 

 the use of that concept, but only those which appear to me 

 to be especially destructive at the present time. The recent 

 work of Bucherer, and the atomic theories of J. J. Thomson 



