PORTION OF THE COSMOS. THE SOLAR DOMAIN. 357 



respecting the colours which the darkened moon should 

 exhibit, according to the different hours at which the eclipse 

 commences ( 573 ). 



In the long-continued controversy respecting the proba- 

 bility or improbability of an atmospheric envelope to the 

 Moon, exact observations of occultations of stars have shown 

 that no refraction takes place at the Moon's edge; and 

 Schroter's assumptions ( 574 ) of a lunar atmosphere and lunar 

 twilight are thus refuted. The comparison of the two 

 values which may be derived for the Moon's semi-dia- 

 meter, on the one hand from direct measurement, and on 

 the other from the duration of the occupation of a fixed 

 star, (i. e. the length of time which the Moon takes to 

 pass over the star), shows that, at the moment when the 

 Moon's limb comes in contact with the star, the starlight 

 is not deflected from its rectilinear course by an amount 

 sensible to our eyes. If there were refraction at the margin 

 of the Moon's disk, the second determination of the semi- 

 diameter must come out less than the first by twice the 

 amount of such refraction ; whereas it has been found, 

 by repeated trials, that the two determinations agree 

 so nearly that it has not been possible to find any 

 decided difference between them ( 5 ? 5 ) . The immersion or 

 disappearance of a star behind the Moon, which can be 

 observed with particular precision on the dark margin, 

 takes place suddenly, and without any gradual diminution 

 of the star's brightness ; and the same is the case with the 

 emersion or reappearance of a star. The few exceptions 

 which have been remarked may have had for their cause 

 accidental changes in our own atmosphere. 



If, then, the Moon is without any gaseous envelope, the 



