282 COSMOS. 



The fiequent occurrence of contrasted colours, constitutes an 

 extremely remarkable peculiarity of multiple stars. Struve, in 

 his great work 17 published in 1837, gave the following results 

 with regard to the colours presented by six hundred of the 

 brighter double stars. In 375 of these, the colour of both 

 principal star and companion was the same and equally in- 

 tense. In 101, a mere difference of intensity could be dis- 

 cerned. The stars with perfectly different colours were 120 

 in number, or one-fifth of the whole; and in the remaining 

 four-fifths the principal and companion stars were uniform 

 in colour. In nearly one-half of these six hundred, the 

 principal star and its companion were white. Among those 

 of different colours, combinations of yellow with blue (as in 

 t Cancri), and of orange with green, (as in the ternary star 

 y Andromedse,) 18 are of frequent occurrence. 



Arago was the first to call attention to the fact that the 

 diversity of colour in the binary systems principally, or at 

 least in very many cases, has reference to the complementary 

 colours the subjective colours, which when united form 

 white." It is a well known optical phenomenon that a faint 



" Struve, Mensurce microm., pp. Ixxvii to Ixxxiv. 



18 Sir John Herschel, Outlines of Astr., p. 579. 



19 Two glasses, w r hich exhibit complementary colours, when 

 placed one upon the other, are used to exhibit white images 

 of the sun. During my long residence at the Observatory 

 at Paris, uiy friend very successfully availed himself of this 

 contrivance, instead of using shade glasses to observe the 

 sun's disc. The colours to be chosen are red and green, 

 yellow and blue, or green and violet. " Lorsqu'une lumi- 

 ere forte se trouve aupres d'une lumiere faible, la derniere 

 prend la teinte complementaire de la premiere. C'est la le con- 

 traste; mais comme le rouge n'est presque jamais pur, on peut 

 tout aussi bien dire que le rouge est complementaire du bleu. 

 Les couleurs voisines du spectre solaire se substituent." 

 ' When a strong light is brought into contact with a feeble 

 one, the latter assumes the complementary colour of the for- 



