108 COSMOS. 



ors — the subjective colors, which, when united; form white. 5 * 

 It is a well known optical phenomenon that a faint white 

 light appears green when a strong red light is brought near 

 it, and that a white light becomes blue when the stronger 

 surrounding light is yellowish. Arago, however, with his 

 usual caution, has reminded us of the fact that even though 

 the green or blue tint of the companion star is sometimes the 

 result of contrast, still, on the whole, it is impossible to deny 

 the actual existence of green or blue stars. t There are in- 



* Two glasses, which exhibit complementary colors when placed one 

 upon the other, are used to exhibit white images of the sun. During 

 my long residence at the Observatory at Paris, my friend very success- 

 fully availed himself of this contrivance, instead of using shade glasses 

 to observe the sun's disk. The coloi's to be chosen are red and green, 

 yellow and blue, or green and violet. " Lorsqu'une lumiere forte se 

 trouve aupres d'une lumiere faible, la derniere prend la teinte comple- 

 mentaire de la premiere. C'est la le contraste; mais comme le rouge 

 n'est presque jamais pur, on peut tout aussi bien dire que le rouge est 

 compUmentaire du bleu. Les couleurs voisines du spectre solaire se 

 substitueut." " When a strong light is brought into contact with a 

 feeble one, the latter assumes the complementary color of the former. 

 This is the effect of contrast ; but as red is scarcely ever pure, it may 

 as correctly be said that red is the complementary of blue : the colors 

 nearest to the solar spectrum reciprocally change." (Arago, MS. of 

 1847.) 



t Arago, in the Connaisance des Temps pour Van 1828, p. 299-300; 

 and in the Annuaire pour 1834, p. 246-250; pour 1842, p. 347-350: 

 " Les exceptions que je cite, prouvent que j'avais bien raison en 1825 

 de n'introduire la notion physique du contraste dans la question des etoi- 

 les doubles qu'avec la plus graude reserve. Le bleu est la couleur re- 

 elle de certaines etoiles. II resulte des observations recueillies jusqu'ici 

 que le firmament est non seulement parseme de soleils rouges etjaunes, 

 comme le savaient les anciens, ma isencore de soleils bleus et verts. 

 C'est au tems et a des observations futures a nous apprendre si les etoi- 

 les vertes et bleues ne sont pas des soleils deja en voie de decroissance ; 

 si les differentes nuances de ces astres n'indiquent pas que la combustion 

 s'y opere a difierens degres ; si la teinte, avec exces de rayons les plus 

 refrangibles, que presente souvent la petite etoile, ne tiendrait pas a la 

 force absorbante d'une atmosphere que developperait Taction de i'etoile, 

 ordinairement beaucoup plus brillante, qu'elle accompagne." "The 

 exceptions I have named proved that in 1825 I was quite right in the 

 sautious reservations with which I introduced the physical notion of 

 contrast in connection with double stars. Blue is the real color of cer 

 tain stars. The result of the observations hitherto made proves that 

 the firmament is studded not only with red and yellow suns (as was 

 known long ago to the ancients), but also with blue and green suna. 

 Time and future observations must determine whether red and blue 

 stars are not suns, the brightness of which is already on the wane; 

 whether the varied appearances of these orbs do not indicate the de- 

 gree of combustion at work within them ; whether the color and the 

 excess of the most refrangible rays often presented by the smaller of 

 two stars be not owing to the absorbing force of ar atmosphere devel 



