4gO LECTURE XLI. 



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 it be possible, on account of their immense distance from us, to distin- 

 guish two such bodies from each other. It follows also, on the same sup- 

 position 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 con- 

 centrated 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 probability, to changes 

 which are perpetually taking place in the atmosphere, and which aflfect its 

 refractive density. It is said that in some climates, where the 'air is re- 

 markably 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 

 than the imperfection of the instrument. Dr. Herschel has observed in the 

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

 tius and Dr. Halley have argued that their number must be absolutely infi- 

 nite, in order that all of them may remain at rest by the opposition of attrac- 

 tions 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 

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



