1820.] Dr. James Bradley, 323 



without interruption ; and in 1727 he enabled others to enjoy 

 the fruit of his researches by pubhshing his theory of the aber- 

 ration of the fixed stars, one of the most useful and ingenious 

 discoveries of modern astronomy. 



It had been long observed that the position of the stars under- 

 goes certain variations which do not in the least correspond 

 with the apparent motion of one degree in 72 years, which is 

 produced by the precession of the equinoxes. 



The Abbe Picaid had remarked these variations in the pole 

 star as early as 1671, but he had neither attempted to reduce 

 them to a constant rule, nor to assign any cause for them. The 

 very numerous observations of Dr. Bradley presented him not 

 merely with the variations observed by Picard, but with many 

 others which had not so much as been suspected. He met 

 with some stars which appeared during the course of a year to 

 change their longitude without any alteration in their latitude ; 

 others appeared to alter their latitude without any alteration in 

 their longitude ^ while others, and this was the case with the 

 greatest number, appeared to describe a small ellipse more or 

 less elongated. 



The amiual period which all these movements, so dift'erent 

 from each other, affected, soon led to the inference that the 

 motion of the earth was intimately connected with the pheno- 

 mena. But the difficulty was to explain in what way it could 

 produce such effects. The first attempts of Dr. Bradley to obtain 

 an explanation were unsuccessful, liut his perseverance was at 

 last crowned with success, and enabled him to discover that all 

 these apparent motions in the stars were the result of the succes- 

 sive motion of lisfht combined with that of the earth round the 



sun. 



It had been long beheved that the velocity of hght was, physi- 

 cally speaking, infinite. M. Roemer was the first who ventured 

 to aiffirm that this opinion was inaccurate, and even to assign the 

 time which light takes to traverse the diameter of the earth's 

 orbit. He had observed that the emersion of the first satellite of 

 Jupiter became later and later in proportion as Jupiter became 

 further and further removed from the opposition ; and that this 

 retardation in an eclipse the nearest possible to the conjunction 

 amounted to 1 1 minutes. He was of opinion that these 11 

 minutes constituted the time that the first ray of the satellite, 

 when it emerged, took in traversing the distance between the 

 two positions of the earth, when near the opposition and near 

 the conjunction; and consequently that the velocity of light is 

 not merely finite, but measurable. 



However reasonable this explanation is now esteemed, it was 

 then thought too bold ; and it was not till long after the death 

 of Roemer that astronomers unanimously agreed that the motion 

 of light was successive. It Avas from this successive motion 

 that Dr. Bradley obtained the explanation of the irregular varia- 



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