Astronomical Society. 371 



of 12^-feet focus, formed, indeed, upon the same general plan of 

 Molyneux's, but furnished with a divided arc of 6| degrees on each 

 side of the zenith point, for the purpose of enabling him to ascertain, 

 by direct observation, whether other stars than y Draconis, would be 

 similarly affected. The instrument's situation, when adjusted, " might 

 be securely depended upon to half a second," and its telescope could 

 be directed to " not less than 12 stars, bright enough to be seen in 

 the day-time" throughout the year: the same changes were observed 

 as had been previously detected with Molyneux's instrument. In- 

 flexible, however, in his resolution not to generalise till sufficient 

 means were collected to lead him to a " probably just conclusion," 

 the year of probation was suffered to be completed before " the ob. 

 servations were examined and compared :" then it was that he satis- 

 fied himself of the general laws of thephsenomena; and then, and not 

 till then, did he endeavour to find out their cause. Convinced that 

 the apparent motion of the stars which he had observed was not owing 

 to nutation — persuaded that a change in the direction of the plumb- 

 line with which the instrument was rectified, was insufficient to have 

 occasioned it — and having appealed unsuccessfully to refraction, — he 

 perceived " that if light was 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 the object ; and that when the eye is moving in 

 different directions, the apparent place of the object would be differ- 

 ent." He therefore announced his discovery in these words : " That 

 all the phaenomena proceeded from- the progressive motion of light and 

 the earth's annual motion in its orbit," or, as he afterwards called it, 

 aberration of light. 



But he who determined its existence determined also its constant, 

 and fixed it at 20"; giving us, therefore, the interval of time in which 

 light travels from the sun to the earth, as eight minutes and seven se- 

 conds, dift'ering from that deduced by Roemer nearly three minutes 

 of time, a circumstance not at all to the discredit of Roemer, consider- 

 ing the imperfect knowledge of the theory of Jupiter's satellites at 

 the time he made his important discovery. 



The observations, however, which led Bradley to the discovery of 

 aberration, and to the determination of its constant, being as yet un- 

 published, have given rise to insinuations certainly ungenerous, and 

 probably unjust. Impelled by more honourable feelings, our illustrious 

 associate Bessel, alluding to the observations of y Draconis made by 

 Bradley when the sector was removed to Greenwich, says*: " Cjelerum 

 Bradleii observationes Wansteadiana; liberari possunt k sectoris mu- 

 tabilitate, quum s<E])ius, eodem tempore, observatae sint stellae, in 

 quibus abcrrationi contraiia fuerunt signa : qua de causa, et propter 

 observationum jirsestantiam, optabile cssct rejjerire ipsa Bradleii 

 autogra|)ha." It will therefore be highly grateful to him, and to 

 astronomers in general, to be informed from this chair, that the 

 manuscripts of the Wanstcad observations arc found ! — that, to the 



* KMnclHintnta Astronomia;, p. l!iJ4, 



3 B 2 honour 



