T \1 PHILOSOPHICAL TRANSACTIONS. [aNNO 17Q(5. 



IX. On the Method of Observing the Changes that happen to the Fixed Stars ; 

 with some Remarks on the Stability of the Light of our Sun. To which is 

 added, a Catalogue of Comparative Briglilness, for ascertaining the Perma- 

 nency of the Lustre of Stars. By If'illiam Herschel, LL. D., F. R. S. p. l66. 

 The earliest observers of the stars have taken notice of their different de- 

 grees of brilliancy, and, by way of expressing their ideas to others, have classed 

 them into magnitudes. Brightness and size among the stars were taken as 

 synonymous terms, and may still be used as such with sufficient truth, though 

 the latter it seems can only be considered as the consequence of the former. 

 The brightest stars were called of the 1st magnitude; the next of the 2d; and 

 those of inferior lustres of the 3d, 4th, and 5th magnitudes; and so on. 



Among the stars of the first '2 or 3 classes tiiere seems to be some natural 

 limit which confines them to a particular order. If we suppose the stars to be 

 about the size of our sun, and at nearly an equal distance from us and from 

 each other, those which form the first inclosure about us will appear brighter 

 than. the rest, and there can be only a small number of them. This hypothesis 

 is nearly confirmed by observation, as may be seen by looking over a globe, 

 and applying a pair of compasses opened to 6o degrees, which should be the 

 angle subtended by the stars of the first magnitude, if they were all scattered 

 equally. For it will be found that the distances from Lyra to Arcturus; from 

 Arcturus to Regulus; from Kegulus to Sirius; from Sirius to )3 Navis; from 

 Elgeuse to Canopus; from Canopus to a Centauri; from a Centauri to Acher- 

 nar; from Acliernar to « Crucis; from Procyon to Canopus; from Fomalhaut 

 to Altair; and from Altair to Antares, agree sufficiently well with this hypo- 

 thesis. It must also be remembered that a perfect equality in the mutual an- 

 guhir distribution of the stars that form the first inclosure, is a thing that is 

 mathematically impossible, and therefore not to be looked for. This would 

 authorize us to take in other intervals, such as from Arcturus to Antares ; from 

 Elgeuse to Regulus ; from Achernar to Rigel ; from Rigel to Capella ; from 

 Ca|)ella to Sirius; from Regulus to Spica ; from Spica to x Crucis ; and from 

 Rigel to Castor ; all which concur, in a great measure, to support the same 

 hypothesis. But as the distribution and real magnitude of stars is not the pre- 

 sent subject, what has been mentioned will be sufficient. 



A 2d layer of stars will be more extensive ; for the superficies of the celestial 

 regions allotted for the situation of these successive stars exceeds the former in 

 the ratio of 4 to 1. And on looking over the collection of stars which astrono- 

 mers have pointed out as belonging to the 2d class, we find that their number 

 is proj)ortionally larger. A similar way of considering the stars of the 3d order 

 miglit be applied, if it dul not already appear, from what has been said of the 

 former '2 orders, when strictly compared with the state of the heavens, that such 



