OF THE COSMOS. COLOUR OF THE FIXED STARS. Ill 



of interference and of diffraction, as we learn from Arago's 

 and Airy's observations. The smallest objects which can 

 still be distinctly seen (telescopically) as luminous points 

 (multiple stars such as Lyrse, and the 5th and 6th star 

 discovered by Struve in 1826 and by Sir John Herschel in 

 the trapezium, ( 2n ) formed by the quadruple star Oriouis) 

 may be employed to test the quantity of light and merits in 

 other respects of optical instruments, whether refractors or 

 reflectors. 



A difference of colour in the proper light of the fixed stars, 

 as well as in the reflected light of the planets, has been re- 

 cognised from very early times ; but the knowledge of this 

 remarkable phenomenon has been wonderfully enlarged 

 since tbe period of telescopic vision, and especially since 

 double stars have become an object of lively interest and 

 diligent observation. We do not here refer to the change 

 of colour which, as has been already remarked, acccompanies 

 scintillation even in the whitest heavenly bodies, still less 

 the transitory and most frequently reddish tinge which the 

 light of stars receives in the vicinity of the horizon from the 

 medium (/. e. the atmospheric strata) through which we 

 view them ; but we speak of the white or of the coloured 

 light which radiates from stars as the result of peculiar lu- 

 minous processes and the particular character of the surface 

 of each. The Grecian astronomers knew only red stars ; 

 while, by the aid of the telescope, the moderns have 

 discovered in the starry vault, in the celestial fields which 

 light traverses, as in the corollas of our flowering plants, and 

 in the metallic oxides, almost every gradation of prismatic 

 colour between the two extremes of refrangibility, or 



