SIS On the Constitution of the Luminiferojis ^ther. 



200,000 miles in a second : we should expect, a priori, the 

 velocity of propagation of normal vibrations to be incompa- 

 rably greater. This is just the conclusion to which we are led 

 quite independently, from dynamical principles of the greatest 

 generality, combined with the observed pha>nomena of optics*. 



I take this opportunity of making a few remarks on my 

 explanation of aberration (Phil. Mag., vol. xxvii. p. 9), more 

 especially as Professor Challis's words at page 168 of the pre- 

 sent volume would naturally lead to the idea, which, however, 

 I believe was not intended, that I had only explained the 

 change in the direction of the normal to a wave of light, so 

 that something w.as wanting to complete, on the suppositions 

 adopted, the explanation of aberration. To prevent misap- 

 prehension, I would observe, that in the explanation of aber- 

 ration I here include the explanation of the rectilinear propa- 

 gation of light, if the explanation of aberration be divided into 

 two parts ; the first, the explanation of rectilinear propagation ; 

 the second, the explanation of aberration on the assumption 

 of rectilinear propagation. To my own mind, the undulatory 

 theory cannot be said to explain aberration unless it explains, 

 either rectilinear propagation, or what is equivalent to it; for 

 had the stars never been observed, I should have thought it 

 excessively improbable that the path of a ray was rectilinear 

 in the neighbourhood of the earth. As to the necessity for 

 an explanation of aberration on any theory of light, I quite 

 agree with Professor Challis, as lie has stated. Indeed, if I 

 ever appeared to differ from him on this point, it was not be- 

 cause I held a different opinion, but because I failed to catch 

 his meaning. 



In my first paper on aberration, it is true that I did not 

 invesdgate the nature of the path of a ray of light in space ; 

 but this was only because the method I employed did not 

 require any such investigation. I showed that, on the sup- 

 positions adopted, the path of a ray, not in space but relatively 

 to the earth, was in the immediate neighbourhood cf the ob- 

 server directed to the a})parent place of the heavenly body 

 from which it came, that is, its place as affected by the ob- 

 served aberration ; and that was sufficient for my purpose. 

 My explanation was not even deficient in consequence of not 

 taking account of the light coming from the wire to which the 

 star was referred ; for according to my method, everything 

 was reduced to the case in which the earth and the aether in 

 its immediate neighbourhood are supposed to be at rest ; so 



• See the introduclion to an admirable memoir by Green, On the Re- 

 flexion and Refraction of Light. — Cambridge Philosophical Transactions, 

 vol. vii. p. 1. 



