PORTION OF THE COSMOS. THE SOLAR DOMAIN. 26 I 



outer one should be a hundred times the Earth's distance 

 from the Sun, this would still not be an eighth part of the 

 distance of the aphelion of the above mentioned comet, or the 

 2200th part ( 46 ) of that from which we should have to 

 view the reflected light of a planet or satellite revolving 

 round a Centauri. But it may be asked, is the assumption 

 ^f the existence of planets or satellites revolving round the 

 fixed stars unconditionally necessary. If we glance at the 

 subordinate particular systems within our general planetary 

 system, we find, notwithstanding the analogies which may 

 be presented by those planets round which many satellites 

 revolve, that there are also other planets, Mercury, Yenus, 

 and Mars, which have not even a single satellite. If we 

 pass from what is simply possible and confine ourselves to 

 what has been actually investigated, we shall be vividly 

 impressed by the idea that the solar system, especially as 

 the last ten years have disclosed it to us, affords the fullest 

 picture of easily recognised direct relations of many cos- 

 mical bodies to one central one. 



In the astronomy of measurement and calculation, the 

 more limited space of the planetary system, by reason of 

 this very limitation, offers, as compared with the considera- 

 tion of the heaven of the fixed stars, incontestable advan- 

 tages in respect to the evidence and certainty of the results 

 obtained. Much of sidereal astronomy is simply contem- 

 plative ; it is so in regard to star-clusters and nebulae, and 

 also the very insecurely grounded photometric classification 

 of the fixed stars. The best assured and most brilliant 

 department in astrognosy, and which in our own time has 

 received such exceeding improvement and enlargement, is 



