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



just before it enters the eye, is the true direction of the object, 

 atmospheric refraction not being considered. 



It seems probable also that th's is the direction in which 

 the object is seen ; if so, the point j» is seen out of its true place. 

 This, however, is not ; n essential consideration. 



I think it important to remark, that the foregoing explana- 

 tion of aberration rests on no hypothesis whatever, being a 

 strict deduction from ascertained facts, without reference to any 

 theory of light. The cause assigned for aberration is, there- 

 fore, a vera eausa, which consequently excludes every explana- 

 tion of a hypothetical kind, such, for instance, as that which 

 Mr. Stokes proposed in the July number of this Magazine. 



Aberration being explained in this manner, it is interesting 

 to inquire whether a proposed theory of light be consistent 

 with this explanation. The object of such an inquiry would 

 be to test the truth of the proposed theory. The only condi- 

 tion the theory is required to fulfil, in addition to that of tem- 

 poraneous transmission, is, that the light from an object tra- 

 verse, just before it enters the eye, a straight line directed to 

 the true position of the object. 



The above condition is satisfied on the theory of emission, 

 because according to that theory light passes from the object 

 to the eye in a straight line. In the undulatory theory, the di- 

 rection of transmission of light is the direction of transmission 

 through space of a given point of a wave in a given phase of 

 vibration. Where the aether is undisturbed, this direction is 

 normal to the front of the wave. Where the aether is in mo- 

 tion, it is the direction resulting from the composition of the 

 motion of propagation of the wave with the motion of trans- 

 lation of the aether. It is easy, therefore, to determine, for a 

 given motion of translation of the aether, the angle which the 

 normal to the front of the wave makes with the direction of 

 transmission of light. In the figure, let//« (not necessarily 

 equal nor parallel to e 1 e) represent the motion of the aether, 

 p' e representing the velocity of light ; then e n is the direction 

 of the normal to the wave that enters the eye at e. If the 

 normal underwent no angular deviation the whole distance 

 from the object to the eye, en would also be the direction of 

 the object, and consequently aberration on this theory would 

 not be accounted for. I gave Mr. Stokes the credit pi having 

 first shown that the normal is shifted through a certain angle 

 as the wave is propagated through the aether set in motion by 

 the earth, and by reasoning as he has done, and supposing 

 certain analytical conditions, which I shall speak of presently, 

 to be satisfied, the deviation is found to be from p 1 towards », 

 exactly through the angle p 1 en. Consequently op* is the di- 



