declination of some of thefxed stars. 43 
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 a. 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 perfect 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 i",5. Now these three stars, but par- 
ticularly the two latter, are among those that have been most 
assiduously observed by Bradley and myself, at each of the 
three periods. Let us suppose 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 exempt 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 
3 5 degrees, for instance, of that point. Upon this supposition, 
how can we possibly reconcile the great errors that must 
* This has been confirmed by subsequent observation. 
