26 THE SKY. 



the stars scattered over the vault of heaven there are 

 many which to the naked eye appear single, but which 

 when seen through the telescope prove to be two stars 

 closely approaching each other. These " double stars " 

 are of two kinds, " optical " and " physical ; " optical 

 double stars are those which appear to be near each other 

 merely from the accident of one being placed behind 

 the other, nearly in a straight line, although in reality 

 at an immence distance and in no way connected. Physical 

 double stars (usually called " binary systems ") consist of 

 two suns comparatively near each other and revolving about 

 their common centre of gravity. Sir William Herschell 



FIG. 25. 



first discovered their physical connection, and thereby 

 proved that the great law of gravitation was not confined to 

 our system, but was the ruling power which controlled and 

 regulated other systems. Astronomers have calculated the 

 orbits of only fourteen such " binary systems " at present 

 with any degree of certainty ; about a thousand double stars 

 are known, but only about one hundred binary systems." In 

 some "binary systems" the two suns are of different colours; 

 white and purple, red and green, or yellow and blue. If 

 any planets revolve about such suns, when situated between 

 the two, what strange phenomena must occur ! Imagine a 

 day during which a red sun had tinted everything crimson, 

 being succeeded at sunset (not, as with us, by darkness) by 

 the rising of a green .sun, changing the colour of every object 

 from red to green, and how curious and beautiful must be 



