56 Presidential Address 



lag. That is certain; and observation 

 of the acceleration or retardation is 

 made by aid of Jupiter's satellites. We 

 have there the dial of a clock, to or from 

 which we advance or recede periodically. 

 It gains while we approach it, it loses 

 while we recede from it, it keeps right 

 time when we are stationary or only 

 moving across the line of sight. 



But then of course it does not matter 

 whether Jupiter is standing still and we 

 are moving, or vice versa: it is a case of 

 relative motion of matter again. So it 

 is if we observe a Doppler effect from the 

 right and left hand limbs of the rotating 

 sun. True, and if we are to permit no 

 relative motion of matter we must use a 

 terrestrial source, clamped to the earth 

 as our receiver is. And now we shall 

 observe nothing. 



But not because there is nothing to 

 observe. Lag must really occur if we 

 are running away from the light, even 

 though the source is running after us at 

 the same pace: unless we make the 

 assumption true only for corpuscular 



