ON THE FIXED STARS. 389 



, The expanse of the universe is strewed, at immense distances, with de- 

 tached portions of a substance, which we suppose to be matter, constituting 

 stars, or suns, planets, and comets ; bodies which certainly agree with each 

 other in the power of emitting or reflecting light, and which, in all proba- 

 bility, have many other properties in common. Such of these as emit 

 then- own light, are called fixed stars ; and this appears to be the only 

 criterion that we can apply to a star : for the word fixed is only to be 

 understood in a comparative sense. 



The stars must necessarily shine by their own light ; for if we grant that 

 they consist of gravitating matter, it must be allowed that no star could be 

 near enough to another to be seen by reflected light, without a very sensible 

 change of the places of both in consequence of their mutual gravitation, nor 

 would Jfbe possible, on account of their immense distance from us, to dis- 

 tinguish two such bodies from each other. It follows also, on the same 

 supposition of the universality of the force of gravity, that the form of the 

 stars must be nearly spherical. 



The light of the stars appears to the naked eye to be generally white ; 

 being too faint to excite the idea of a particular colour ; but when it is 

 concentrated by Dr. Herschel's large speculums, it becomes, in various 

 stars, of various hues ; and indeed to the naked eye some of the stars 

 appear a little redder and others a little bluer. The cause of the twinkling 

 of the stars is not fully ascertained, but it is referred, with some proba- 

 bility, to changes which are perpetually taking place in the atmosphere, 

 and which affect its refractive density. It is said that in some climates, 

 where the air is remarkably serene, the stars have scarcely any appearance 

 of twinkling.* 



Above two thousand stars are visible to the naked eye ; and when a tele- 

 scope is employed, their number appears to increase without any other 

 limit then the imperfection of the instrument. Dr. Herschel has observed 

 in the milky way above ten thousand stars in the space of a square degree. 

 Lucretius and Dr. Halley t have argued that their number must be abso- 

 lutely infinite, in order that all of them may remain at rest by the oppo- 

 sition of attractions acting in every possible direction ; but we are by no 

 means certain that they do remain in perfect equilibrium. 



Of the actual magnitude of the stars we can give no exact account ; but 

 they are divided into seven or more orders, according to the degrees of their 

 apparent brightness. There is, however, reason to suppose, from the quan- 

 tity of light emitted by the brightest stars, that some of them are much 

 larger than the sun. Those stars which are below the sixth magnitude are 

 scarcely visible without the help of telescopes. The distances of all the stars 

 from us and from one another are so great, as not to be capable of being 

 immediately compared with their diameters ; for no star subtends an angle 

 large enough to be ascertained by direct observation. The more perfect the 

 instruments that we employ, the smaller are the apparent diameters of the 

 fixed stars. Dr. Herschel found that one of the stars of the first magnitude, 

 when viewed in his best telescopes, appeared to be about one third of a 



* See Garcia, Hist, et Mem. 1743, H. 28 ; and Michell, Ph. Tr. 1767, p. 234. 

 f Ph. Tr. 1720, xxxi. 22. 



