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XXXII. On the Principles to be applied in explaining the 

 Aberration of Light. By the Rev. J. Challis, M.A., Plu- 

 mian Professor of Astronomy in the University of Cam- 

 bridge*. 



THE aberration of light having been brought before the 

 notice of the readers of this Journal by several recent 

 communications, I am unwilling to let the subject drop with- 

 out saying a few more words respecting the principles to be 

 applied in the explanation of the phenomenon, which possibly 

 may appear, after all that has been said, to be involved in un- 

 certainty. I propose to answer the question, Is the aberration 

 of light to be attributed to known causes, or must we, to ex- 

 plain it, have recourse to hypothesis ? 



The first attempts to explain aberration referred it to the 

 combined effect of the motion of the earth and the tempora- 

 neous transmission of light, and accordingly proceeded on the 

 principle of attributing it to known causes. It must, however, 

 be admitted that every attempt to show how the observed 

 effect resulted from these causes, what was the particular 

 modus operandi, v/as unsatisfactory. Some idea appropriate 

 to the subject was still wanting. This idea I consider that I 

 have succeeded in supplying. I have argued, as had not been 

 argued before, that because the direction of a celestial object 

 is necessarily referred to the direction of a terrestrial object, 

 light from the one as well as light from the other must be 

 taken account of in considering the question of aberration. 

 It is self-evident, that if at any instant two objects appear in 

 the same direction, whatever course the light from the more 

 distant may have taken before it reaches the nearer, it subse- 

 quently pursues a common course with light from the latter, 

 and the two portions of light enter the eye at the given instant 

 simultaneously. The direction in which the light comes is 

 therefore judged to be the same as the direction at that instant 

 of the nearer object from the eye. But during the interval the 

 light takes to pass from the nearer or terrestrial object to the 

 eye, this object is carried by the earth's motion away from the 

 direction of the progression of light, and the two directions, 

 at the time they are judged to be coincident, are in reality 

 separated by a certain angle. This angle is aberration. I 

 may refer to my communication in the February Number for 

 a proof, which I venture to say is as cogent as any proof in 

 the elements of geometry, that according to the principles just 

 stated, an astronomical instrument employed to measure the 

 earth's way, as it is called, would measure a smaller angle. 



* Communicated by the Author. 



