VOL. LXXXVI,] PHILOSOPHICAL TRANSACTIONS. 7 15 



and from 6 to 7 m. In all these inferior orders less than an alteration of a mag- 

 nitude and a half could hardly deserve attention. 



Here we have supposed all references to be made to the same author ; for 

 when other astronomers are consulted, the uncertainty is much increased. A 

 star which in Flamsteed's catalogue stands 1.2 m, may be found 2 m, in another 

 author : 2 m in the former may be rated 2.3 m, or even 3 m by the latter. Of 

 course 3 m and 4 m may be written for the magnitude of the same star by dif- 

 ferent persons. 4 and 5 m as well as 5 and 6 m are frequently interchanged, 

 and no stress can be laid on such nominal differences in different catalogues. 

 We can hardly allow less than half a magnitude in the higher orders, and a 

 whole one in the inferior classes for this uncertainty. 



To apply what has been said : suppose there should be some inducement to 

 believe a certain star, such as |3 Leonis, to have changed its lustre. Now hav- 

 ing no real, existing type of comparison, we can only refer to the general ima- 

 ginary one; and here the rules we have laid down will be of considerable service. 

 The magnitude of this star given by Flamsteed is 1 .2 m ; but as we have shown 

 that there is some ground to admit that 1 .2 m, even in this coarse way of refer- 

 ence, may be distinguished from what the same author seems to have taken for 

 2 m, we conclude that the star has probably lost some of its former brightness. 

 Again, he gives j3 1.2 m, and y 2 m. This notation may be taken to imply, 

 though indirectly, that p is larger than y ; which not being tlie case, we have 

 an additional reason to suspect a change. De La Caille sets down (3 2 m. Now 

 the difference between the notation 1.2 m of Flamsteed and 2 m of the latter 

 author, can add nothing to the force of the argument for a change ; as we have 

 observed before, that a considerable allowance must be made for nominal varie- 

 ties in different authors. Nor can we draw any support from the magnitude 

 itself, because the star will pass very well for one of that order, when compared 

 with other stars which are marked 2 m by the same author. But when De La 

 Caille marks |3 2 m, and y 3 m, we may then conclude that he estimated j3 to be 

 larger than y, though we do not know that he compared these two stars toge- 

 ther ; because a whole magnitude in the 2d class, as we have said, cannot well 

 be mistaken, coarse as is the type to which the reference is made. On the 

 whole therefore, we conclude that j3 Leonis is now less brilliant than it was 

 formerly. 



In this manner, with proper circumspection, we may get at some certainty, 

 even by the method of magnitudes ; the imperfection of it however in other 

 cases is very obvious, a- Leonis, for instance, being marked by Flamsteed 4.5 m, 

 the star itself will in every respect pass for one of that magnitude, when com- 

 pared to a mental standard taken from other stars of the same author. Nor can 

 its being brighter than stars which have a magnitude of a superior lustre affixed 



4 Y 2 



