Prof. Powell on the Theory of the Aberration of Light. 437 



refrangibility of light, the ravibow \s perfectly explained with- 

 out reference to the undulatory or any other theory of the 

 nature of light. Yet it is material to the truth of that theory 

 that it should 'explain the dispersion; and if it did so, as it 

 confessedly explains the laws of reflexion and refraction, it 

 would Explain the rainbow, which is a mere consequence from 

 them. 



In other words, the aberration is not an independent pro- 

 perty of light, like refraction or polarization, which require a 

 theory to explain them, but the result (if the above principles 

 be admitted) of simpler known causes. Any theory which ex- 

 plains these causes explains the aberration. But it is a mat- 

 ter of importance for the credit of any theory, that it should 

 be able to explain them. Thus, more precisely, the question 

 is not whether any theory explains the aberration, but whether 

 it accounts for the facts from which the aberration is a conse- 

 quence. 



On the other hand, there are some, who, not entirely falling 

 in with the above explanation, may view the whole matter 

 under a different aspect, and may ask, before coming to any 

 explanations, does not the naked fact of the aberration stand 

 out as a prima facie exception to the strict universality of the 

 law of rectilinear propagation ? and they may argue, as we 

 are quite ignorant of the cause of the rectilinear motion of 

 light, or the modus operandi which produces it, we cannot as- 

 sume that as light approaches the earth in motion, it may not 

 in some way be influenced, or a deviation caused : and it is 

 only so far as we assume some theory of the nature of light 

 that we can form any conception which may guide us to a 

 conclusion on this point : and assuredly a theoretical invests 

 gation would in this point of view be not only desirable for 

 the theory, but necessary for the explanation of the fact. 



Those then who contend for the necessity for a theoretical 

 investigation to explain the aberration, appear implicitly to 

 assume that we cannot infer the absolute universality of the law 

 of rectilinear propagation of light at all parts of its course to 

 the earth in motion ; though the result of the investigation on 

 the undulatory hypothesis, with the aether in motion, is to 

 establish it. 



The question then is, whether this assumption be really 

 called for in the nature of the reasoning : Is our ignorance of 

 the nature and propagation of light so entire, that, for anything 

 we know to the contrary, the mere circumstance of the rapid 

 motion of the observer may in some unknown way act upon 

 the ray of light as it approaches him and divert its course 



