394 Rev. J. Challis on the Aberration of Light. 



page 91 of the February Number of the Philosophical Maga- 

 zine, does not even suppose the existence of an aether. On 

 the contrary, Mr. Stokes's rests both on the hypothesis of an 

 aether and on a gratuitous and very particular supposition 

 respecting its motion. By Mr. Stokes's admission, I have 

 shown on my principles, that if to the earth's way, as mea- 

 sured by an astronomical instrument, be added in the same 

 plane an angle equal to the product of the ratio of the earth's 

 velocity to the velocity of light and the sine of the earth's way, 

 we obtain the direction in which light from a star progresses 

 just before it enters the eye. By measures taken with astro- 

 nomical instruments, it is found that if to the same angle in 

 the same plane be added the product of 20"*42 and the sine of 

 the earth's way, the mean place of the star is obtained. [The 

 numerical quantity is that adopted in the British Association 

 Catalogue of Stars.] Nowit happens that the ratio of the earth's 

 velocity to the velocity of light is known independently of the 

 above-mentioned measures, by observations of the eclipses of 

 Jupiter's satellites. Delambre states {Abrege d' Astronomie, 

 p. 493), that by very exact and extensive researches on the 

 satellites of Jupiter, he found for this ratio 20"'25. The close 

 approximation of these numerical values justifies me in con- 

 cluding that the light from the star enters the eye, quam 

 proxime, in the direction of a line drawn to the eye from the 

 star's mean place ; or, in Mr. Stokes's notation, that s 2 coin- 

 cides very nearly with s. Mr. Stokes appears to be dissatis- 

 fied because this inference is not deduced by theory alone. 

 I conceive that it is not the less certain because it is deduced 

 from facts; and as Mr. Stokes does not contend that it is not 

 true, I need say no more on this point. 



The "confession" which Mr. Stokes says that I made, I 

 am ready to make again. I allow that, anterior to the above 

 comparison with the result of astronomical measures, it could 

 not be anticipated that aberration would be wholly accounted 

 for by the motion of the earth and the finite velocity of light, 

 without reference to any theory of light. The comparison 

 shows that it is so accounted for, and the inevitable conse- 

 quence is, that any explanation which rests on a hypothetical 

 motion of the aether, must hejictitious. 



I really think that I have now said quite enough in defence 

 of a very unexceptionable piece of reasoning, and if Mr. 

 Stokes should have anything further to urge, I must decline 

 answering it. 



I am, Gentlemen, 



Cambridge Observatory, Your obedient Servant, 



April 11, 1846. J. CHAILis 



