Prof. Cballis on the Cause of the Aberration of Light. 53 



That these propositions having been proved, show that the 

 heat produced by chemical combination, or the expansion occa- 

 sioned by it among the particles of bodies, differs in nothing from 

 that occasioned by the contraction of like particles ; that in both 

 cases the resulting heat or expansion in other bodies is merely 

 the necessary effect, or rather accompaniment of the contraction 

 going on in the combining or contracting ones. 



And that the analogy between the approximating of particles 

 in chemical combination, and that of those of a cooling body, 

 extends also to other particulars. 



VIII. On the Cause of the Aberration of Light. 

 By Professor Challis. 

 To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 



IN page 568 of the Supplementary Number of the Philoso- 

 phical Magazine for December, the following passage occurs 

 in an exti'act from the Comptes Rendus of the French Academy 

 for September 29, 1851 : — "Many hypotheses have been pro- 

 posed to account for the phenomena of aberration in accordance 

 with the doctrine of undulations. Fresnel, in the first instance, 

 and more recently Doppler, Stokes, Challis, and many others, 

 have published memoirs on this important subject ; but it does 

 not seem that any of the theories proposed have received the 

 entire assent of physicists. In fact, the want of any definite 

 ideas as to the properties of the luminous aether and its relations 

 to ponderable matter, has rendered it necessary to form hypo- 

 theses," &c. As it might be supposed from this statement, that, 

 in common with others, I had attempted to account for the 

 aberration of light on certain hypotheses respecting the sether, I 

 beg permission to say a few words for the purpose of correcting 

 bucIi a misapprehension. 



In what I have written on aberration, I have expressly main- 

 tained that the phenomenon may be explained by known facts, 

 without making any hypothesis whatever, and independently of 

 any theory of light. As all that is essential in the explanation 

 I nave given on these principles admits of being condensed 

 within a very small compass, and as by exhibiting it I shall best 

 attain the object of this Note, I propose to reproduce it here, 

 after making one preliminary remark. The first attempts to 

 explain aberration took account of the course of the ray, and only 

 our point on its course which partakes of the earth's motion, 

 namely, tin- eye of the spectator. The explanation I have pro- 

 poaed takea account of two such points. This addition, however 

 simple it may appear, removes all the obscurity attaching to the 

 original explanations. The two points selected were the eye' of 

 the spectator, or rather the point of the eye through which the 



