SKCT. xxxvii.] COLOURS 'OF THE STARS. 21 



nearest fixed star is from the sun, the velocity of the 

 revolving star must exceed the power of imagination to 

 conceive. The elliptical motion of the double stars shows 

 that gravitation is not confined to the planetary motions, 

 but that systems of suns in the far distant regions of the 

 universe are also obedient to its laws. The stellar systems 

 present a kind of sidereal chronometer, by which the chro- 

 nology of the heavens will be marked out to future ages by 

 epochs of their own, liable to no fluctuations from such 

 disturbances as take place in our system. Some stars are 

 apparently double, though altogether unconnected, one 

 being far behind the other in space, as a Lyrse, which 

 apparently consists of two stars, one of the first, the other 

 of the eleventh magnitude. Aldebaran, a Aquilee, and 

 Pollux are remarkable instances of these optically double 

 stars. 



The double stars are of various hues, but they most fre- 

 quently exhibit the contrasted colours. The large star is 

 generally yellow, orange, or red ; and the small star blue, 

 purple, or green. Sometimes a white star is combined with 

 a blue or a purple, and more rarely a red and white are 

 united. In many cases these appearances are due to the 

 influence of contrast on our judgment of colours. For ex- 

 ample, in observing a double star, where the large one is a 

 full ruby red, or almost blood colour, and the small one a 

 fine green, the latter loses its colour when the former is hid 

 by the cross wires of the telescope. Still there are a vast 

 number of instances where the colours are too strongly 

 marked to be merely imaginary. Sir John Herschel ob- 

 serves, in one of his papers in the Philosophical Transactions, 

 as a very remarkable fact, that, although red stars are com- 

 mon enough, no example of a solitary blue, green, or purple 

 star has yet been produced. 



Sirius is the only star on record whose colour has changed. 

 In the time of Ptolemy it was red ; now it is one of the 

 whitest stars in the heavens. 



