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XXVI. On the Course of a Baij of lAglit from a Celestial Bod if 

 to the Earth's Stirface, according to the Hijpothcsis of Undu- 

 lations. By the Rev, J. Challis, M.A., F.R.A,S., Plumian 

 Professor qf Astronomy in the University of Cambridge"^. 



THE question I am about to consider arises out of the 

 explanation of the aberration of light which I gave in 

 the course of a discussion in the pages of this Journal, of 

 which Professor Powell has given an account in vol. xxix. 

 p. 425, and vol. xxx. p. 93, That explanation was based on the 

 fact, that the measured direction of a body not j)artaking of the 

 earth's ^notion is nccessarihj referred to the direction of a body 

 partaking of the earth's motion. From this principle, combined 

 with the known velocity of the earth, and the velocity of light 

 deduced from observations of the eclipses of Jupiter's satellites, 

 not only does aberration result (Phil. Mag., vol. xxvii. p. 321, 

 and vol. xxviii. p. 91), but the calculated amount agrees also 

 as closely as possible with that ascertained by astronomical 

 measurement (Phil. Mag., vol. xxviii. p. 394). There is there- 

 fore no residual phaenomenon to be accounted for. The ex- 

 planation requires no consideration of the way in which the 

 eye is acted upon by light, it rests on no hypothesis whatever, 

 being a strict deduction from facts, it remains the same whether 

 we adopt the emission theory of light or the undulatory, 

 whether there be asther or no aether, whether the aether be in 

 motion or at rest. I make this statement, lest it should be 

 supposed that the aberration of light depends for its explana- 

 tion in any degree upon the answer to the theoretical question 

 I now proceed to consider. The truth of the undulatory 

 theory of light depends very materially upon it. 



The aberration of light being accounted for in the manner 

 referred to above, it follows as a necessary consequence, that 

 the course of light from a celestial object to the eye of the 

 spectator is rectilinear. This is easily conceivable on the 

 emission theory of light; but is the fact consistent with the 

 undulatory theory? The aetherial medium must be put in mo- 

 tion, first, by the earth's rotation about its axis, next by its 

 motion in its orbit, and lastly, by the motion in space which 

 it partakes of in common with the other bodies of the solar 

 system. Is it probable that through the aether thus disturbed 

 a wave can be propagated in a rectilinear course? It has, in 

 fact, been proved by Mr. Stokes (Phil. Mag., vol. xxvii. p. 9), 

 that the normal to the front of the wave will be bent through 

 a certain small angle in a certain direction, inclining towards 

 that in which the earth is moving; and if nothing further 

 , * Coniniunicated by the Author. 



