24 RELATIVITY 



them to compare notes to find out which of them is 

 wrong, and why? Presently they return and say: "It 

 was your fault because in one respect your instructions 

 were not explicit. You did not mention what motion 

 the scale should have when it was being used." One 

 of them without thinking much about the matter had 

 kept the scale at rest on the earth. The other had 

 reflected that the earth was a very insignificant planet of 

 which the Professor had a low opinion. He thought it 

 would be only reasonable to choose some more impor- 

 tant body to regulate the motion of the scale, and so he 

 had given it a motion agreeing with that of the enor- 

 mous star Betelgeuse. Naturally the FitzGerald contrac- 

 tion of the scale accounted for the difference of results. 



I am disinclined to accept this excuse. I say severely, 

 "It is all nonsense dragging in the earth or Betel- 

 geuse or any other body. You do not require any 

 standard external to the problem. I told you to measure 

 the distance of two points on the blackboard; you should 

 have made the motion of the scale agree with that of 

 the blackboard. Surely it is commonsense to make your 

 measuring scale move with what you are measuring. 

 Remember that next time." 



A few days later I ask them to measure the wave- 

 length of sodium light — the distance from crest to crest 

 of the light waves. They do so and return in triumphal 

 agreement: ''The wave-length is infinite". I point out 

 to them that this does not agree with the result given 

 in the book (.000059 cm.). "Yes", they reply, u we 

 noticed that; but the man in the book did not do it 

 right. You told us always to make the measuring scale 

 move with the thing to be measured. So at great trouble 

 and expense we sent our scales hurtling through the 

 laboratory at the same speed as the light." At this speed 



