THE MOON. 487 



without gradual diminution of the star's brilliancy ; just 

 so the egress or reappearance of the star. In the case of the 

 few exceptions which have been described, the cause may 

 have consisted in accidental changes of our atmosphere. 



If, however, the Earth's Moon is destitute of a gaseous 

 envelope, the stars must appear then, in the absence of all 

 diffuse light, to rise upon a black sky ;** no air-wave can 

 there convey sound, music, or language. To our imagination, 

 so apt presumptuously to stray into the unfathomable, the 

 Moon is a voiceless wilderness. 



The phenomenon of apparent adherence on and within the 

 Moon's edge,** sometimes observed in the occultation of stars, 

 can scarcely be considered as a consequence of irradiation, 

 which, in the narrow crescent of the Moon, on account of the 

 very different intensity of the light in the ash-coloured part 

 of the Moon, and in that which is immediately illuminated by 

 the Sun, certainly makes the latter appear as if surrounding 

 the former. Arago saw during a total eclipse of the Moon, a 

 star distinctly adhere to the slightly luminous disc of the 

 Moon during the conjunction. It still continues to be a 

 subject of discussion between Arago and Plateau whether the 

 phenomenon here mentioned depends upon deceptive percep- 

 tion and physiological causes,* 4 or upon the aberration of 



** M'adler, in Schumacher's Jahrbuch fur 1840, p. 188. 



13 Sir John Herschel (Outlines, p. 247) directs attention to 

 the ingress of such double stars as cannot be seen separately 

 by the telescope, on account of the too great proximity of the 

 individual stars of which they consist. 



** Plateau, Sur I' Irradiation, in the Mem. de I'Acad. Royalc 

 des Sciences et Belles- Lettres de Bruxelles, torn. xi. p. 142, 

 and the supplementary volume of Poggendorffs Annalen, 

 1842, pp. 79-128, 193-232, and 405 and 443. " The probable 

 cause of the irradiation is an irritation produced by the light 

 upon the retina, and spreads a little beyond the outline of the 

 image.'* 



