98 SIR G. GABRIEL STOKES, BART,, F.R.S.; ON 



passage of the heavenly bodies — the planets for instance — 

 through it. Yet there appears to be no certain indication of 

 any such resistance. It has been observed indeed in the 

 case of Encke's comet, that at successive revolutions the 

 comet returned to its perihelion a little before the calculated 

 time. This would be accounted for by the supposition that 

 it experienced a certain amount of resistance from the ether. 

 Although at first sight we might be disposed to say that 

 such a resistance would retard perihelion passage, yet the 

 fact that it would accelerate it becomes easily intelligible, if 

 we consider that the resistance experienced would tend to 

 check its motion, and so prevent it from getting away so far 

 from the sun at aphelion, and would consequently bring it 

 more nearly into the condition of a planet circulating round 

 the sun in a smaller orbit. 



Many years ago I asked the highest authority in this 

 country on Physical Astronomy, the late Professor Adams, 

 what he thought of the evidence afforded by Encke's comet 

 for the existence of a retarding force, such as might arise 

 from the ether. He said to me that he thought we did not 

 know enough as to whether there might not possibly be a 

 planet or planets within the orbit of Mercury which would 

 account for it in a different way. But quite independently 

 of such a supposition it is worthy of note that the remarkable 

 phenomena presented by the tails of comets render it by iio 

 means unlikely that even witliout the presence of a resisting 

 medium, and without the disturbing force arising from the 

 attraction of an unknown planet situated so near to the sun 

 as not to have been seen hitherto, the motion of the head of 

 a. comet might not be quite the same as that of a simple 

 body representing the nucleus, and being subject to the 

 gravitation of the sun and planets and nothing else. It 

 appears that the tails consist of some kind of matter driven 

 from the comet with an enormous velocity by a sort of 

 repulsion emanating from the sun. If the nucleus loses in 

 this manner at each perihelion passage an exceedingly 

 small portion of its mass, which is repelled from the sun, it 

 is possible that the residue may experience an attraction 

 towards the sun over and above that due to gravitation, and 

 that possibly this may be the cause of the observed accelera- 

 tion in the time of passing perihelion even though there be 

 no resistance on the part of the ether. So that the question 

 of resistance or no resistance must be left an open one. 



The supposition that the ether would resist in this manner 



