M 210-213] Aberration 265 



measure of the velocity of light in terms of the dimensions 

 of the earth's orbit, the determination of aberration being 

 susceptible of considerably greater accuracy than the 

 corresponding measurements required for Roemer's method. 



211. One difficulty in the theory of aberration deserves 

 mention. Bradley's own explanation, quoted above, refers 

 to light as a material substance shot out from the star or 

 other luminous body. This was in accordance with the 

 corpuscular theory of light, which was supported by the 

 great weight of Newton's authority and was commonly 

 accepted in the i8th century. Modern physicists, however, 

 have entirely abandoned the corpuscular theory, and regard 

 light as a particular form of wave-motion transmitted 

 through ether. From this point of view Bradley's ex- 

 planation and the physical illustrations given are far less 

 convincing; the question becomes in fact one of considerable 

 difficulty, and the most careful and elaborate of modern 

 investigations cannot be said to be altogether satisfactory. 

 The curious inference may be drawn that, if the more 

 correct modern notions of the nature of light had prevailed 

 in Bradley's time, it must have been very much more 

 difficult, if not impracticable, for him to have thought of his 

 explanation of the stellar motions which he was studying ; 

 and thus an erroneous theory led to a most important 

 discovery. 



212. Bradley had of course not forgotten the original 

 object of his investigation. He satisfied himself, however, 

 that the agreement between the observed positions of y Dra- 

 conis and those which resulted from aberration was so 

 close that any displacement of a star due to parallax which 

 might exist must certainly be less than 2", and probably 

 not more than |", so that the large parallax amounting to 

 nearly 30", which Hooke claimed to have detected, must 

 certainly be rejected as erroneous. 



From the point of view of the Coppernican controversy, 

 however, Bradley's discovery was almost as good as the ? 

 discovery of a parallax ; since if the earth were at rest 

 no explanation of the least plausibility could be given of 

 aberration. 



213. The close agreement thus obtained between theory 

 and observation would have satisfied an astronomer less 



