SECT. XXXVI. COLOURS OF THE STARS. 401 



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

 of the eleventh magnitude. Aldebaran, a Aquilre, and Pollux are 

 remarkable instances of these optically double stars. It has been 

 shown how favourable that circumstance is for ascertaining the 

 parallax of the nearest of the two. (N. 232.) 



The double stars are of various hues : sometimes both stars 

 are of the same colour, as in Centauri and 61 Cygni, where the 

 larger stars are of a bright orange and the smaller ones a deeper 

 tint of the same, but they most frequently 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 

 example, 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. That is the case with y Andromeda}, 

 which is a triple star, the small one, which appears green, 

 being closely double, i Cancri is an instance of a u large yellow 

 star and a small one which appears blue by contrast. Still there 

 are a vast number where the colours are decidedly different, and 

 suggest the curious idea of two suns, a red and a green, or a 

 yellow and a blue, so that a planet circulating round one of them 

 may have the variety of a red day and a green day, a yellow day 

 and a blue day. Sir John Herschel observes, in one of his papers 

 in the Philosophical Transactions, as a very remarkable fact, 

 that, although red stars are common 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. 



M. Struve has found that, out of 596 bright double stars, 375 

 pairs have the same intensity of light and colour ; 101 pairs 

 have different intensity, but the same colour ; and 120 pairs 

 have the colours of the two stars decidedly different. 



Certain rays, which exist in the sun's light, are wanting in the 

 spectra of every coloured star, and probably never existed in the 

 light of these stars, as there is no reason to believe that they 

 are absorbed by the stars' atmosphere, though they may be by 



