148 COSMOS. 



that moment in which it touches the Moon's edge. If a re- 

 fraction took place at the edge of the Moon, the second de- 

 termination of her diameter must give a value smaller by 

 tw^ice the amount of the refraction than the former ; but, on 

 the contrary, both determinations correspond so closely in 

 repeated determinations, that no appreciable difference has 

 ever been detected."=^ The ingress of stars, which may be 

 particularly well observed at the dark edge, takes place 

 suddenly, and without gradual diminution of the star's brill- 

 iancy ; 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 oui 

 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 ;t no air- wave can 

 there convey sound, music, or language. To our imagina- 

 tion, 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-colored 

 part of the Moon, and in that which is immediately illumin- 

 ated by the Sun, certainly makes the latter appear as if sur- 

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

 Moon, a star distinctly adhere to the slightly luminous disk 

 of the Moon during the conjunction. It still continues to be 



* Bessel, TJeher eine angenommene Atmosphdre des Mondes in Schu- 

 macher's Asiron. Nachr., No. 263, p. 416-420. Compare also Beer and 

 Madler, Der Monde, § 83 and 107, p. 133 and 153; also Arago, in the 

 Annuaire for 1846, p. 346-353. The frequently mentioned proof of the 

 existence of an atmosphere round the Moon, derived from the greater or 

 less perceptibility of small superficial configurations and "the Moon- 

 clouds moving round in the valleys," is the most untenable of all, on 

 account of the continually-varying condition (darkening and bi'ighten- 

 ing) of the upper strata of our own atmosphere. Considerations as to 

 the form of one of the Moon's horns qn the occasion of the solar eclipso 

 on the 5th of Septepber, 1793, induced William Herschel to decide 

 against the assumption of a lunar atmosphere. (Pkilos. Transact., vol. 

 ixxxiv., p. 167.) 



t Madler, in Schumacher's Jahrbuch for 1840, p. 188. 



j Sir John Herschel (Outlines, p. 247) directs attention to the ingress 

 of such double stars as can not be seen separately by the telescope, on 

 account of the too great proximity of the individual stars of which they 

 consist. 



