Rev. J. Challis on the Aberration of Light. 177 



The difference, or aberration, is readily calculated from know- 

 ing by observations of the eclipses of Jupiter's satellites, the 

 ratio of the earth's velocity to the velocity of light. Being so 

 calculated the amount is found to be the same as the amount of 

 aberration independently determined by astronomical observa- 

 tion. It follows from this accordance, not only that the aber- 

 ration of light is entirely accounted for on these principles, 

 but also, as a corollary, that the direction of the progression 

 of light from a star, as it enters the eye, is the true direction of 

 the star. Whether it be the star, or the terrestrial object 

 to which it is referred, that is seen in its true place, is a curious 

 question, not readily answered, and not in the least degree ne- 

 cessary to be answered in the present inquiry. 



Sufficient reasons have now, I think, been adduced for 

 coming to the conclusion, that the question I proposed to 

 consider must receive the following categorical answer: — The 

 aberration of light is entirely due to known causes, viz. the 

 motion of the earth and the temporaneous transmission of 

 light, and does not require for its explanation any hypothesis 

 whatever. 



What then becomes of the theories which have been framed 

 to account for aberration on the hypothesis of certain motions 

 of the aethereal medium ? As explanations of aberration they 

 can be of no value, it being an acknowledged principle in phi- 

 losophy, that an hypothesis is not to be sought for to explain 

 what may be explained by known causes. All that is left for 

 the theorist to do, supposing, as it appears necessary to sup- 

 pose, that the aether is in some way put in motion by the 

 motion of the earth, is to show that no aberration results from 

 such motion, the whole being attributable to the earth's mo- 

 tion. This problem I have considered in my two former 

 communications, not because it was necessary to do so to 

 complete the explanation of aberration, but with the view of 

 removing an objection that might be raised against the undu- 

 latory theory of light. By taking account both of the light 

 from the star and the light from the terrestrial object to which 

 the star's direction is referred, I found that no aberration 

 would result from the motion of the tether, provided it satis- 

 fied certain not improbable analytical conditions. A different 

 conclusion would be arrived at by the same reasoning, if the 

 light from the star, as is commonly done in treating of aber- 

 ration, were alone considered. 



With these remarks I dismiss the subject of aberration, 

 having attained the object I had in view in taking it up, if I 

 have succeeded in extricating the explanation of the phaeno- 

 menon from hypothesis and conjecture, and placing it on its 

 true basis. 



Cambridge Observatory, Feb. 17, 1846. 

 Phil. Mag. S. 3. Vol. 28. No. 18C. March 184.6. O 



