182 Mr. K Campbell on the ^Ether. 



nature of radiation and on the proof that the principle of 

 relativity is an adequate foundation for electromagnetic 

 theory, but it is clear that such theories do not provide either 

 a sufficient or a necessary reason for abandoning the con- 

 ception. Sir J. J. Thomson, the author of the earliest and 

 most far-reaching atomic theory of radiation, devoted much 

 of his presidential address before the British Association to a 

 description of the properties of the aether, while, on the 

 other hand, I hope to show that a consideration of no ideas 

 more novel than the elements of electrostatics may lead to 

 grave doubts concerning the utility of that conception. If 

 both sides could be induced to express their views in detail, 

 the difference between them would be found to concern the 

 fundamental principles of scientific knowledge rather than 

 more special problems of observation and intuition. Perhaps 

 it is because men of science exhibit considerable shyness in 

 discussing the essential foundations of their study that there 

 has been so little direct attack on or defence of the concep- 

 tion of the aether. The following remarks may help in some 

 measure towards a thorough consideration of the whole of 

 this important problem *. 



§ 2. We must first inquire what is meant by " the aether," 

 and why it was ever invented. Almost the only definition of 

 the conception with which I am acquainted is that of the late 

 Lord Salisbury, who described it as " the subject of the verb 

 ' to undulate/ " It is not immediately obvious why that 

 verb requires a special subject, but a very little consideration 

 will make out a case which is at least prima facie plausible. 

 The principle of the conservation of energy is perhaps the 

 only proposition that is accepted by all physicists as a neces- 

 sary basis for their science, and the maintenance of that 

 principle would seem at first sight to require some such 

 conception as the aether. When a body is radiating energy 

 to another at a lower temperature separated from it by a 

 finite distance, there is a finite interval of time during which 

 energy has been lost by the first body and has not been 

 gained by the second ; if the energy is not to be regarded as 

 lost altogether for that interval, it might seem that it must 

 be regarded as gained by some third body which is neither 

 the source nor the receiver. This body, the body which is 

 the vehicle of the undulatory energy of light, is the aether. 



The development of the electromagnetic theory of light 



* Note.— It may be pointed out that the gist of the argument is con- 

 tained in chap. xiv. of 'Modern Electrical Theory' (Cambridge, 1907), 

 and in an article in the ' New Quarterly Review ' No. 3. 



