1892.] on the Motion of the Ether near the Earth. 569 



source in its true position. It would be otherwise for the case of pro- 

 jectiles from a revolving turret. 



Thus, neither translation of star nor rotation of sun can affect 

 direction. There is no aberration so long as the receiver is sta- 

 tionary. 



But what about a wind, or streaming of the medium past source 

 and receiver, both stationary ? Look at Fig. 1 again. Suppose a row 

 of stationary cannon firing shots, which get blown by a cross wind 

 along the slant 1 A Y (neglecting the curvature of path which would 

 really exist) : still the hole in the target fixes the gun's true position, 

 the marker looking along Y A sees the gun which fired the shot. 

 There is no true deviation from the point of view of the receiver, 

 although the shots are blown aside and the target is not hit by the 

 particular gun aimed at it. 



With a moving cannon, combined with an opposing wind, Fig. 1 

 would become very like Fig. 2. 



(N.B. — The actual case, even without complication of spinning, 

 &c., but merely with the curved path caused by steady wind-pressure, 

 is not so simple, and there would really be an aberration or apparent 

 displacement of the source towards the wind's eye : an apparent ex- 

 aggeration of the effect of wind as shown in the diagram.) 



In Fig, 2 the result of a wind is much the same, though the details 

 are rather diff'erent. The medium is supposed to be drifting down across 

 the field opposite to the arrows. The source is stationary at S. The 

 arrows show the direction of waves in the medium ; the dotted slant 

 line shows their resultant direction. A wave centre drifts from D to 

 1 in the same time as the disturbance reaches A, travelling down the 

 slant line D A. The angle between dotted and full lines is the angle 

 between ray and wave movement. Now, if the motion of the medium 

 inside the receiver is the same as it is outside, the wave will pass straight 

 on along the slant to Z, and the true direction of the source is fixed. 

 But if the medium inside the 



target or telescope is stationary, Fxg. 4. 



the wave will cease to drift as ^ 



soon as it gets inside, under cover 

 as it were ; it will proceed along 

 the path it has been really pur- 

 suing in the medium all the time, 

 and make its exit at Y. In this 

 latter case, of different motion of 

 the medium inside and outside 

 the telescope, the apparent direc- 

 tion, such as Y A, is not the true 

 direction of the source. The ^^y through a Moving Stratum. 



ray is in fact hent where it enters 

 the differently-moving medium (as shown in Fig. 4). 



A slower moving stratum bends an oblique ray, slanting with the 

 motion, in the same direction as if it were a denser medium. 

 Vol. XIII. (No. 86.) 2 q 



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