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



by the direction of the telescope, manifestly implying that the 

 light which comes down the telescope in motion coincides with 

 that from the star at rest, it must be allowed, approach much 

 nearer to a complete explanation, though they do not expli- 

 citly give it. It is true, several of these writers refer to specu- 

 lations of another kind, which would maintain the same con- 

 clusion with regard to the naked eye, by supposing two im- 

 pacts upon it, respectively proportional to the velocities of 

 light and the earth, the resultant of which coincides with the 

 direction of the ray. But they give such reasoning as entirely 

 separate from the former: and Vince (p. 199, 8vo.) in par- 

 ticular censures those writers who would confound the two. 

 Such reasonings are obviously insufficient as explanations of 

 the aberration, as well from the uncertainty of the analogy 

 assumed between the effects of light and those of mechanical 

 impact, as on other grounds. 



But by far the most lucid and satisfactory was the view of 

 the subject taken by Clairaut*. In his memoir he gave per- 

 haps the first complete investigation of the astronomical 

 theory, leading to formulas for the aberration, not only in de- 

 clination but in right ascension also, Bradley having originally 

 considered only the former f : and it is as the foundation of this 

 investigation that he introduces his well-known illustration, so 

 deservedly adopted by most subsequent writers, and which is 

 peculiarly worthy of consideration in relation to our present 

 object. 



Fig. 1. Fig. 2. * 



* Mtm. Acad. Paris, 1737, p. 205. + Rigmid's Memoir, p. xxxiii. 



