488 COSMOS. 



sphericity and refrangibility of the eye. 85 Those cases In 

 which it has been asserted that a disappearance and reap- 

 pearance, and then a repeated disappearance, have been 

 observed during an occupation, may probably indicate the 

 ingress to have taken place at a part of the Moon's edge 

 which happened to be deformed by mountain declivities and 

 deep chasms. 



The great differences in the reflected light from particular 

 regions of the illuminated disc of the Moon, and especially 

 the absence of any sharp boundary between the inner edge of 

 the illuminated and ash-coloured parts in the Moon's phases, 

 led to the formation of several very rational theories as to 

 the inequalities of the surface of our satellite, even at a very 

 remote period. Plutarch says distinctly, in the small but 

 very remarkable work On the Face in the Moon, that we may 

 suppose the spots to be partly deep chasms and valleys, 

 partly mountain peaks, " which cast long shadows, like 



35 Arago, in the Comptes Rendus, torn. viii. 1839, pp. 713 

 and 883. "Les phenomenes d'irradiation signales par M. 

 Plateau sont regardes par M. Arago comme les effets des 

 aberrations de refrangibilite et de sphericite de Fceil, combines 

 avec 1'indistinction de la vision, consequence des circonstances 

 dans lesquelles les observateurs se sont places. Des mesures 

 exactes prises sur des disques noirs a fond blanc et des 

 disques blancs a fond noir, qui etoient places au Palais du 

 Luxembourg, visibles a 1'observatoire, n'ont pas indique les 

 effets de 1'irradiation." ' ' The phenomena of irradiation pointed 

 out by M. Plateau are regarded by M. Arago as the effects 

 of the aberration of sphericity and refrangibility of the eye, 

 combined with the indistinctness of vision consequent upon 

 the circumstances in which the observers are placed. The 

 exact measurement taken of the black discs upon a white 

 ground, and the white discs upon a black ground which were 

 placed upon the palace of Luxembourg, and visible at the 

 Observatory, did not present any phenomena of irradiation/' 



