Bearing of Hotation on Relativity. 71 



cannot find a system that supplies a natural test, and are 

 unable to make one. 



In the foregoing sentences it is, of course, adopted that 

 when a phenomenon occurring on Jupiter is transmitted by 

 light to the Earth, the velocity of its transmission through 

 the intervening space is entirely independent of any drift 

 that Jupiter may possess relative to the aether ; this drift is 

 shared, let ns say, by the whole solar system, and it becomes 

 an observable quantity at the receiving point — the Earth, 

 because it may change the distance over which the trans- 

 mission through pure aether must take place. 



Returning to the imagined model of two concentric spheres, 

 consider again this bearing upon relative time and its mea- 

 surement, as these appear in the usual exposition of the 

 Principle of Relativity. 



Consider the two observers A and B, imagined at the 

 beginning, and suppose for simplicity that A, B are both 

 on the equator of the uniform relative motion of the spheres. 

 If A, B compare their clocks by light signals, say, it may 

 be shown, as in the works on Relativity, that their zeroes 

 will differ by ux/c 2 , where x is the arc AB, u the velocity of 

 B relative to A, and c the velocity of light. If the relative 

 motion of B is in the same direction as the transmission of 

 the signal, B's time is slow. But in our imagined case, B 

 will receive two signals, generally at different times and 

 always in opposite directions, and might consequently set 

 two clocks by them, one slow and the other fast. These 

 clocks would keep permanently different rates, one losing 

 and the other gaining ru 2 /c 2 in each relative revolution or 

 " day," where t is the length of this " day." But this 

 " day " would itself be an observable phenomenon, possessed 

 in common by both A and B, and marked as the interval 

 between two consecutive coincidences of B with A. In our 

 case, then, B would be able to ascertain that the times kept 

 by his tw r o clocks rated by two light signals were both 

 erroneous, nor would he require to adopt the same numerical 

 measure of the velocity of light ns A, in order to provide 

 himself with a standard measure of time, as he is supposed to 

 do, in the Theory of lielativity. 



It is hardly necessary to point out that our imagined case 

 of time-determination is not an artificial one, but is as close 

 as an ideal construction permits to the actual practice by 

 which the solar day and the sidereal day are found. We 

 may go a little further. It has been the practice to observe 

 at Greenwich the transit of the lunar crater Mosting A. 

 Imagine an observer in the Moon, situated in Mosting A 

 and taking the moment when Greenwich crossed the medial 



