8 Mr. G. G. Slokes on the Aberration of Light. 



sible fluid would ; and when, at the end of this interval, the 

 earth is reduced to rest, the whole of the aether will be re- 

 duced to rest, except as regards an extremely small motion, 

 of the same nature as that already considered in the case of 

 an ordinary fluid. But in the present case this small motion 

 will be propagated into space with the velocity of light; so 

 that just before the commencement of the third interval the 

 aether may be considered as at rest, and everything will be the 

 same as before. Supposing now the number of intervals of 

 time to be indefinitely increased, and their magnitude indefi- 

 nitely diminished, we pass to the case in which the earth is 

 supposed to move continuously. 



It appears then, from these views of the constitution of the 

 aether, that (a.) must be an exact differential, if it be not pre- 

 vented from being so by the action of the air on the aether. 

 We know too little about the mutual action of the aether and 

 material particles to enable us to draw any very probable 

 conclusion respecting this matter; I would merely hazard the 

 following conjecture. Conceive a portion of the aether to be 

 filled with a great number of solid bodies, placed at intervals, 

 and suppose these bodies to move with a velocity which is very 

 small compared with the velocity of light, then the motion of 

 the aether between the bodies will still be such that (a.) is an 

 exact differential. But if these bodies are sufficiently close 

 and numerous, they must impress either the whole, or a con- 

 siderable portion of their own velocity on the aether between 

 them. Now the molecules of air may act the part of these 

 solid bodies. It may thus come to pass that («.) is an exact 

 differential, and yet the aether close to the surface of the earth 

 is at rest relatively to the earth. The latter of these condi- 

 tions is however not necessary for the explanation of aberra- 

 tion. 



There is one curious consequence of the theory contained 

 in my paper of last July, which I did not at the time observe. 

 On referring to this paper, it will be seen that if the motion 

 of the aether is such that (a.) is an exact differential, the 

 change in the direction of the normal to a wave of light, as 

 the wave passes from a part of space where the disturbance 

 of the aether due to the motion of the earth is insensible to 

 another part where the disturbance is sensible, is given by the 

 equation 



« 2 - a i = y» 



which is what (6.) becomes when a x and v t are each put equal 

 to zero; and the plane passing through the direction of the 



