1892.] 



on the Motion of the Ether near the Earth. 



567 



this line, as shown by the arrows. There are thus two directions to 

 be distinguished. There is the row of successive shots, and there is 

 the path of any one shot. These two directions enclose an angle. It 

 may be called an aberration angle, because it is due to the motion of 

 the source, but it need not give rise to any aberration. True direction 

 may still be perceived from the point of view of the receiver. Attend 



Fig. 1. 



Disturbances with Momentum. 



to the target. The first shot is supposed to be entering at A, and if 

 the target is stationary will leave it at Y. A marker looking along 

 Y A will see the position whence the shot was fired. This may be 

 likened to a stationary observer looking at a moving star. He sees it 

 where and as it was when the light started on its long journey. He 

 does not see its present position, but there is no reason why he should. 

 He does not see its physical state or anything as it is now. There is 

 no aberration caused by motion of source. 



But now let the receiver be moving at same pace as the gun, as 

 when two grappled ships are firing into each other. The motion of 

 the target carries the point Y forward, and the shot A leaves it at Z, 

 because Z is carried to where Y was. So in that case the marker 

 looking along Z A will see the gun, not as it was when firing, but as 

 it is at the present moment ; and he will see likewise the row of shots 



Fig. 2. 



Y 



Disturbances without Momentum. 



making straight for him. This is like an observer looking at a terres- 

 trial object. Motion of the earth does not disturb ordinary vision. 

 Fig. 2 shows as nearly the same sort of thing as possible for the 



