190 Mr. N. Campbell on the ^Ether. 



and Planck (the latter recently developed by Stark * so as 

 to resemble the former very closely) will be found very 

 difficult for believers in the aether to assimilate or to explain 

 away ; if they attempt to do so it will doubtless be in the 

 belief that the concept of the aether is worth retaining. A 

 demonstration that the case for the aether is ludicrously 

 weak, where it was thought to be strongest, that the concept 

 has never been the source of auything but fallacy and con- 

 fusion of thought, may serve to expedite its relegation to 

 the dust-heap where now " phlogiston " and " caloric " are 

 mouldering. 



Note. 



It is desirable that a few remarks should be made on the 

 relation between this paper and another on " The Principles 

 of Dynamics," which is published iti the same number of 

 this Magazine ; for it might appear that some statements 

 made above are inconsistent with those made elsewhere. 

 One of these statements is that to which the footnote on 

 p. 184 is appended. In the Ci Principles of Dynamics " it is 

 pointed out that the velocity which is discussed in physics is 

 almost always Velocity, and that it is not definable imme- 

 diately in terms of distances and times. (The notation used 

 in this Note is the same as that used in the paper to which 

 reference is made.) 



I have not reconciled these apparent inconsistencies by 

 adopting the terms employed in the " Principles of Dynamics," 

 which was written considerably later, because it seems to me 

 that the argument as it is stated here, though objectionable/ 

 in form, is more convincing than it would have been other- . 

 wise, and requires less subtlety of thought. But I propose'* 

 in this Note to point out how it would appear if viewe^f r^jn 

 the standpoint of the later ideas. 



The only meaning which is given to the word " velocity " 



in scientific discussion, which can be stated without assuming 



dv 

 the truth of a scientific theory, is -j~ , where r and t are 



Distances and Times bearing relation A to distances and 

 times. Other quantities, like Absolute Velocity, which are 

 called velocities because they are related in a certain way to 

 Relative Velocities, can only be defined by stating the relation 

 in the form of an equation, which is, in fact, the expression 

 of a scientific theory. If we reject the identification of 

 particles of the aether by their energy-content, we reject the 

 possibility of measuring the distance of such a particle from 

 any other particle, and consequently of defining the Relative 



* J. Stark, Phjs. Zeit. Sept. 1909, p. 579. 





