422 STABS THAT HAVE VANISHED. [SECT, xxxvn. 



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 either by the stars or earth's 

 atmosphere. There are no defective rays in the white light 

 of Sirius, Procyon, and others; but Sir David Brewster 

 found in the spectrum of the orange-coloured light of 

 f Herculis a defective band in the red space, and two or 

 more in the blue ; consequently, the orange colour of the 

 star is owing to a want of blue rays ; for flames in which 

 certain rays are wanting take the colour of the predomi- 

 nating rays. If the black rays in the solar spectrum were 

 owing to the absorption of the sun's atmosphere, the light 

 from the margin of his disc, having to pass through a 

 greater thickness of it, would exhibit deeper lines than 

 that which comes from his centre ; but, as no difference is 

 perceptible, it may be inferred that the analogous bands in 

 the light of the coloured stars are not due to the absorption 

 of their atmospheres, but that they arise from the different 

 kinds of combustion by which these bodies are lighted up. 



Many stars have vanished from the heavens ; 42 Virginis 

 seems to be of this number, having been missed by Sir John 

 Herschel on the 9th of May, 1828, and not again found, 

 though he frequently had occasion to observe that part of 

 the heavens. Sometimes stars have all at once appeared, 

 shone with a bright light, and vanished. Several instances 

 of these temporary stars are on record. A remarkable one 

 occurred in the year 125, which is said to have induced 

 Hipparchus to form the first catalogue of stars. Another 

 star appeared suddenly near a Aquilse in the year 389, 

 which vanished after remaining for three weeks as bright 

 as Venus. On the 10th of October, 1004, a brilliant star 



