Common Sense of Relativity. 509 



the physically infinite velocity has actually been measured. 

 The first and third parts of the postulate are scarcely 

 dubitable. 



But perhaps such arguments are unconvincing io the 

 physical instinct, so I proceed to considerations which should 

 overcome any difficulty which is felt in connexion with the 

 great importance attributed to the velocity o£ light when 

 dealing with phenomena which appear to have nothing to do 

 with light. These considerations are based on the fact, 

 obvious when it is pointed out, that such velocities as 

 distinguish practically a disturbed from a quiet system, 

 velocities, that is to say, which are not physically infinitesimal, 

 can only be measured by optical or electrical means. 

 When we hear of the " velocity of the earth rela lively to 

 the sun " or the " velocity of a /3-particle relatively to its 

 source/' association leads us first to think of a quantity 

 measured by the distance travelled relatively to a metre scale 

 during the passage of the hand of a chronometer over a 

 certain part of its face. But a little reflection will show that 

 this is not w T hat we mean by these velocities : we have never 

 held a metre rod up ag;iinst the sun or an electron and 

 observed the change in relative position. It would lead us 

 too far to inquire here what exactly we do mean by such an 

 expression as the " velocity of a /3-particle,''' but it could be 

 shown quite easily that that expression has no meaning what- 

 ever, unless we assume the truth of the fundamental equations 

 of the electron theory *. And since those equations involve 

 the velocity of light, it is not surprising that that quantity 

 enters when we are considering the velocity of an electron. 

 " The velocity of a /3-particle " is called a '* velocity " because, 

 within a certain range of values, the number representing it 

 is the same as the number representing a velocity measured 

 by a scale and clock (as is shown by the Kowland experiment), 

 but, outside the range within which the scale-and-clock 

 measurements of velocity are applicable, " the velocity of an 

 electron " is dependent for its meaning on certain theories. 

 To inquire whether, outside this range, this velocity would 

 agree with that determined by the scale and clock is as absurd 

 as to inquire whether, if all triangles had four sides, all circles 

 would be square. 



* Some remarks on this subject are to be found in a paper on " The 

 Principles of Dynamics," Phil.' Mag. xix. p. 168. 



