PROPER MOTION OF THE STARS. 255 



during the time of. their being visible ." (The luminous pro- 

 cess in them has simply ceased.) " There exist therefore in 

 celestial space dark bodies of equal magnituder, and probably 

 in as great numbers as the stars." u So also Madler, in his 

 Untersuchungen uber die Fixstern-Systeme, says : u " A dark 

 body might be a central body; it might, like our own sun, 

 be surrounded in its immediate neighbourhood only by dark 

 bodies like our planets. The motions of Sirius and Procyon, 

 pointed out by Bessel, force us to the assumption that there 

 are cases where luminous bodies form the satellites of dark 

 masses." 11 It has been already remarked that the advocates 

 of the emanation theory consider these masses as both invisible, 

 and also as radiating light: invisible, since they are of such 

 huge dimensions that the rays of light emitted by them (the 

 molecules of light) being impeded by the force of attraction, 

 are unable to pass beyond a certain limit. 15 If, as may 

 well be assumed, there exist, in the regions of space, dark 

 invisible bodies in which the process of light-producing 

 vibration does not take place, these dark bodies cannot fall 

 within the sphere of our own planetary and cometary system, 

 or at all events their mass can only be very small, since 

 their existence is not revealed to us by any appreciable 

 disturbances. 



The inquiry into the quantity and direction of the motion of 

 ike fixed stars, (both of the true motion proper to them, and 

 also of their apparent motion, produced by the change in the 

 place of observation, as the earth moves in its orbit,) the 



13 Laplace, Expos, du Syst. du Monde, 1824, p. 395. 

 Lambert, in his Kosmologische Briefe, shows remarkable ten- 

 dency to adopt the hypothesis of large dark bodies. 



14 Madler, Untersuch, uber die Fixstern-Systeme, th.ii. (1848), 

 s. 3 ; and his Astronomy, s. 416. 



w Cosmos, vol. iii. p. 117 and note: Laplace, in Zach's 

 Ally. Geogr. Ephem., bd. iv. s. 1 ; Madler, Astr., s. 393. 



