Prof. Powell on the Theory of the Aberration of Light. 435 



siderable source of cavil. The point determined by the posi- 

 tion of the telescope and its wire is that to which the eye refers 

 the star t which is consequently seen out of its true -place by 

 the amount of aberration, while the wire is seen in its true 

 direction. The precise explanation then may be stated in all 

 its essential particulars, divested of the mystery and obscurity 

 which was sometimes found in it, and rendered at once simple, 

 exact, and relieved of all perplexity from appearing at vari- 

 ance with received notions as to the direction of vision. 



But a material part of the question before us relates to the 

 investigation of the aberration in connexion with theories of 

 the nature of light. A considerable portion of the existing 

 controversy (we may observe) has arisen not only on the 

 question of the details of such theoretical analysis, but also on 

 the more general logical question of the grounds on which 

 any such investigation should be undertaken at all. 



As to the actual application of theory to the explanation of 

 the phenomenon, Euler long since, adopting for simplicity 

 the emission theory, admitted that some difference in the view 

 taken of the case would result according to the hypothesis of 

 light adopted*. He more particularly notices (§ 7) the ob- 

 jection which might arise on the emission theory, that light 

 would partake in the velocity of the body from which it was 

 projected, which would not be the case on the undulatory. 

 This is further discussed (§ 24, 25, 28). 



In the present state of the question it seems on all hands to 

 be admitted that the point at issue is a very simple one. It 

 in fact reduces itself ultimately to this: whatever theory we 

 adopt, will it account for the uninterrupted maintenance of 

 the law of the rectilinear propagation of light from a star to 

 the earth in motion, at all parts of its course, but especially 

 near the earth, with the same velocity ? 



The emission theory obviously fulfills this condition; as does 

 also the undulatory, if the aether be at rest. But one of the 

 greatest difficulties attending that theory, as formerly delivered, 

 was the conception of the aether pervading all bodies and the 

 interior of the earth yet remaining at rest, a passage being 

 afforded to it through the interstices of the densest matter, in 

 the words of Dr. Youngf? " as freely as the wind passes 

 through a grove of trees/' while the earth was in rapid mo- 



* Comm. Petrop. xi. 150. 



f Phil. Trans., 180.2, Experiments and Calculations on Physical Optics, 

 § iv . It is remarkable that the author says he was led to this idea chiefly 

 from considering the aberration of the stars. 



