352 COSMOS. 



ellipse which the Earth describes round the Sun, is reflected 

 in a small perfectly similar one, in which the central point of 

 the Sun moves round its own and the Earth's common centre 

 of gravity." In general notices like the present, we need 

 hardly enter into any special consideration of the question 

 as to whether the planetary bodies, among which we must 

 class interior and exterior comets, may not be capable, at 

 least in part, of generating some special light of their own, in 

 addition to that which they receive from the central body. 



We have hitherto acquired no direct evidence of the exist- 

 ence of dark planetary bodies revolving round other fixed 

 stars. The faintness of the reflected light would prevent 

 their ever being visible to us, if, as Kepler conjectured 

 (long before Lambert) such bodies actually revolve round 

 every fixed star. If the nearest fixed star, a Centauri, be 

 226,000 times the Earth's distance, or 7,523 times the 

 distance of Neptune; if a very distant comet, that of 1680 

 (to which has been ascribed, although on very uncertain 

 data, a revolution of 8,800 years), is twenty-eight times the 

 distance of Neptune from our solar system when in its aphe- 

 lion, then the distance of the fixed star a Centauri is still 270 

 times greater than the distance of our solar system from the 

 aphelion of the most remote comet. The light of Neptune 

 is reflected to us from a distance thirty times greater than 

 our distance from the Sun. If, by the future construction of 

 more powerful telescopes, three additional planets should be 

 recognized, each situated at about 100 times the Earth's 

 distance from the other, even this would not amount to the 

 eighth part of the distance intervening to the aphelion of 

 the comet referred to ; or to the 2,200th part of the distance 1 



1 See Cosmos, vol. i. pp. 95, 138, where I based my cal- 

 culations oil the distance of Uranus, winch then constituted 



