Dedinution of some of the principal Fixed Stars. 181 



dieted place, and y Ursce Majoris half a second to the north. 

 Now I am quite at a loss to conceive how this difference in so 

 small an arc can arise from error of observation, and I can 

 only attribute it to that cause, whatever it may be, which seems 

 so generally to depend not on the polar distance, but on the 

 right ascension of the star. 



2. The second group which I shall consider, contains the 

 stars « Arietis, Arcturus, and Aldebaran, comprehended within 

 an arc of about six degrees and a half Of these three, 

 Arcturus alone has yet been observed by reflection; but from 

 the present very jjerfect state of the Greenwich circle, which 

 the method of reflection has enabled me to ascertain, it can- 

 not be doubted that the places of the two other stars are well 

 determined*. In Arcturus the southern deviation is nearly 

 insensible, but in the two other stars it is very considerable, 

 being in each not less than \"'5. Now these three stars, but 

 particularly the two latter, are among those that have been 

 most assiduously observed by Bradley and myself at each of 

 the three periods. Let us supjiose then, if it be possible, that 

 the whole of these deviations arise from error of observation : 

 or, in other words, that no systematic deviation has really taken 

 place in the stars, but that their proper motions are uniform. 

 Then we must admit that the mural quadrant and the mural 

 circle have at each period given the polar distance of Arcturus 

 correct, or at least subject to the same constant error ; and as 

 this star has been observed at each period, at all times of the 

 day, and at all seasons of the year, the observations may be 

 considered as perfectly exemj)t from accidental error. It will, 

 I believe, be readily conceded that both instruments are so far 

 perfect, that if the error be either nothing, or a given quantity 

 at one point of the arc, the errors must be very nearly indeed 

 the same within a moderate distance, as within 15 degrees, 

 for instance, of that point. Upon this supposition, how can 

 we possibly reconcile the great errors that must have been 

 committed in stars, adjacent as to polar distance, but of op- 

 posite right ascensions ? I do not wish to press these remarks, 

 in order to obtain greater confidence than they deserve, for ob- " 

 servations which can never be regarded with too much suspi- 

 cion ; but the arguments I have used appear to me to follow 

 logically from the data before us, and strongly to indicate the 

 probability that some cause purely astronomical has, at least, 

 some share in producing these unexpected deviations. 



3. The third group, a Herculis, u Pegasi, and Regulus^ is 

 still more remarkable, being comprehended within two degrees 



* This has been confirmed by iiibscfjuent objCivatinii, 



of 



