$ 2c8] Aberration 26 1 



own at Wansted, so arranged that it was possible to observe 

 through it the motions of stars other than y Draconis. 



Several stars were watched carefully throughout a year, 

 and the observations thus obtained gave Bradley a fairly 

 complete knowledge of the geometrical laws according to 

 which the motions varied both from star to star and in 

 the course of the year. 



208. The true explanation of aberration, as the pheno- 

 menon in question was afterwards called, appears to have 

 occurred to him about September, 1728, and was published 

 to the Royal Society, after some further verification, early 

 in the following year. According to a well-known story,* ' 

 he noticed, while sailing on the Thames, that a vane on 

 the masthead appeared to change its direction every time 

 that the boat altered its course, and was informed by the 

 sailors that this change was not due to any alteration in 

 the wind's direction, but to that of the boat's course. In 

 fact the apparent direction of the wind, as shewn by the 

 vane, was not the true direction of the wind, but resulted 

 from a combination of the motions of the wind and of the 

 boat, being more precisely that of the motion of the wind 

 relative to the boat. Replacing in imagination the wind 

 by light coming from a star, and the boat shifting its 

 course by the earth moving round the sun and continually 

 changing its direction of motion, Bradley arrived at an 

 explanation which, when worked out in detail, was found 

 to account most satisfactorily for the apparent changes in 

 the direction of a star which he had been studying. His 

 own account of the matter is as follows : 



" At last I conjectured that all the phaenomena hitherto men- 

 tioned proceeded from the progressive motion of light and the 

 earth's annual motion in its orbit. For I perceived that, if light 

 \vas propagated in time, the apparent place of a fixed object 

 would not be the same when the eye is at rest, as when it is 

 moving in any other direction than that of the line passing 

 through the eye and object ; and that when the eye is moving 



* The story is given in T. Thomson's History of the Royal Society, 

 published more than 80 years afterwards (1812), but I have not been 

 able to find any earlier authority for it. Bradley's own account of 

 his discovery gives a number of details, but has UQ allusion to this 



