THE MOON. 147 



mosphere, and thrown into the shadow cone. The reddened 

 or glowing disk is moreover never uniformly colored. Some 

 places always appear darker, and are, at the same time, con- 

 tinually changing color. The Greeks had formed a peculiar 

 and curious theory with respect to the -different colors which 

 the eclipsed Moon was said to present according to the hour 

 at which the eclipse took place.* 



During the long dispute as to the probability or improba- 

 bility of an atmospheric envelope round the Moon, accurate 

 occult observations have proved that no refraction takes 

 place on the surface of the Moon, and. that, consequently, the 

 assumption made by Schroterf of the existence of a lunar 

 atmosphere and a lunar twilight are disproved. " the 

 comparison of the two values of the Moon's diameter which 

 may be respectively deduced from direct measurement, or 

 from the length of time that it remains before a fixed star 

 during the occultation, teaches us that the light of a fixed 

 star is not perceptibly deflected from its rectilinear course at 



qu'elle se trouve plus pres du centre de 1'ombre geometrique ; car se 

 aont les rayons les moins refrangibles qui se propagent le plus abon- 

 damment par diffraction, a mesure qu'on s'eloigne de la propagation en 

 ligne droite." " The diffracted light which penetrates into the Earth's 

 shadow always predominated, and was, indeed, alone sensible. It was 

 the more red or orange in proportion as it was nearer to the geomet- 

 rical center of the shadow ; for those rays which are least refrangible 

 are those which are propagated most abundantly by diffraction, in pro 

 portion as they differ from a rectilinear course." The phenomena of 

 diffraction take place as well in a vacuum, according to the acute in 

 vestigations of Magnus (on the occasion of a discussion between Airy 

 and Faraday). Compare, in reference to the explanations by diffrac 

 tion in general, Arago in the Annuaire for 1846, p. 452-455. 



* Plutarch (De Facie in Orbe Lnnae}, Moral., ed. Wytten., torn, iv., 

 p. 780-783 : " The fjery. charcoal-like, glimmering (dvdpaKoeid^ coloi 

 of the eclipsed Moon (about the midnight hour) is, as the mathemati- 

 cians affirm, owing to the change from black into red and bluish, and 

 is by no means to be considered as a character peculiar to the earthy 

 surface of the planet." Also Dio Cassius (lx., 26, ed. Sturz, p. iii., p 

 779), who occupied himself especially with eclipses of the Moon, and 

 the remarkable edicts of the Emperor Claudius, which predicted the di' 

 mensions of the eclipsed portion, directs attention to the very different 

 colors which the Moon assumed during the conjunction. He says (lxv. r 

 11, torn, iv., p. 185, Sturtz), "Great was the excitement in the camp 

 of Vitellius in consequence of the eclipse which took place that night. 

 The mind was filled with melancholy apprehensions, not so much at 

 the eclipse itself, although that might appear to predict misfortune to 

 an unquiet mind, but much more from the circumstance that the Moon 

 displayed blood-red, black, and other gloomy colors." 



t SchrSter, Selenotopographische Fragmente, th. i., 1791, p. 668; th. 

 ii., 1802, p. 52. 



