118 COSMOS. 



at present attainable in this kind of observation, that the 

 constant of aberration, and consequently the velocity of light, 

 is the same for all fixed stars, the question has frequently been 

 mooted, whether it be not possible that there are luminous 

 cosmical bodies, whose light does not reach us, in conse- 

 quence of the particles of air being turned back by the force 

 of gravitation exercised by the enormous masses of these bodies. 

 The theory of emission gives a scientific form to these imagi- 

 native speculations. 60 I here only refer to such views because 

 it will be necessary in the sequel that we should consider certain 



60 Madler, Astr., s. 380; also Laplace according to Moigno, 

 Repertoire d' Optique moderne, 1847, t. i. p. 72. " Selon la 

 theorie de 1'emission on croit pouvoir demontrer que si le 

 diametre d'une etoile fixe serait 250 fois plus grand que celui 

 du soleil, sa densite restant la nieme, r attraction exercee a sa 

 surface detruirait la quantite de mouvement, de la molecule 

 lumineuse emise, de sorte qu'elle serait invisible a de grandes 

 distances." " It seems demonstrable by the theory of emission 

 that if the diameter of a fixed star be 250 times greater than 

 that of the sun its density remaining the same the attraction 

 exercised on the surface would destroy the amount of motion 

 emitted from the luminous molecule ; so that it would be in- 

 visible at great distances." If, with Sir William Herschcl, 

 we ascribe to Arcturus an apparent diameter of 0"*1, it 

 follows that the true diameter of this star is only eleven times 

 greater than that of our sun. (Cosmos, p. 138.) From the 

 above considerations on one of the causes of non-luminosity 

 the velocity of light must be very different in cosmical bodies 

 of different dimensions. This has, however, by no means been 

 confirmed by the observations hitherto made. Arago says in 

 the Comptes rendus, t. viii. p. 326, " Les experiences sur Tegale 

 deviation prismatique des etoiles, vers lesquelles la terre 

 marche ou dont elle s'eloigne, rend compte de 1'egalite de 

 vitesse apparente de toutes les etoiles." " Experiments made 

 on the equal prismatic deviation of the stars towards which 

 the earth is moving, and from which it is receding, explain 

 the apparent equality of velocity in the rays of all the 

 stars." 



