THE MOON. . 149 



a subject of discussion between Arago and Plateau whether 

 the phenomenon here mentioned depends upon deceptive per- 

 ception and physiological causes,* or upon the aberration of 

 sphericity and refrangibility of the eye.f Those cases in 

 which it has been asserted that a disappearance and reap- 

 pearance, and then a repeated disappearance, have been ob- 

 served during an occultation, may probably indicate the in- 

 gress 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 disk of the Moon, and especially 

 the absence of any sharp boundary between the inner edge 

 of the illuminated and ash-colored 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 Mount 

 Athos, whose shadow reaches Lemnos."$ The spots cover 

 about two fifths of the whole disk. In a clear atmosphere, 

 and under favorable circumstances in the position of the 



* Plateau, Sur V Irradiation, in the M6m. de V Acad. Royale des Sci- 

 ences et Belles-Leltres de Bruxelles, torn, xi., p. 142, and the supplement- 

 ary volume of Poggendorff's Annalen, 1842, p. 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." 



t Arago, in the Comptes Rendus, torn, viii., 1839, p. 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 1'ceil, combines avec 1'indistinction de la vision, conse- 

 quence 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 etaient places au Palais du Luxembourg, 

 visibles a 1'obsei-vatoire, n'ont pas indique les eflfets 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 refrangi- 

 bility 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 disks upon a white ground, and the 

 white disks upon a black ground, which were placed upon the palace 

 of Luxembourg, and visible at the Observatory, did not present any 

 phenomena of irradiation." 



t Plutarch, Moral, ed. Wytten., torn, iv., p. 786-789. The shadow 

 of Athos, which was seen by the traveler Pierre Belon (Observations de 

 Singularity trouvtes en Grece, Asie, etc., 1554, !iv. i., chap. 25), reached 

 the brazen cow in the market-town Myrine in Lemnos. 



