THE SUN. 469 



That part of the Moon's edge which was not projected 

 upon the Sun's disc again became perceptible, especially 

 during the egress. 4 



4 This outline of the Moon, clearly perceived by four 

 observers during the total eclipse of the Sun on the 8th of 

 July, 1842, was never previously described as having been 

 seen during similar eclipses. The possibility of seeing an 

 exterior outline, appears to depend upon the light which is 

 given by the third outermost envelope of the Sun and the 

 ring of light (corona). " La Lune se projette en par tie sur 

 Tatmosphere du Soleil. Dans la portion de la lunette ou 

 1'image de la Lune se forme, il n'y a que la lumiere provenant 

 de 1'atmosphere terrestre. La Lune ne fournit rien de sen- 

 sible, et, semblable a un ecran, elle arrete tout ce qui provient 

 de plus loin et lui correspond. En dehors de cette image, et 

 precisement a partir de son bord, le champ est eclaire d lafois 

 par la lumiere de 1'atmosphere terrestre et par la lumiere de 

 1'atmosphere solaire. Supposons que ces deux lumieres 

 reunies forrnent un total plus fort de -fa que la lumiere atnio- 

 spherique terrestre, et, des ce moment, le bord de la Lune sera 

 visible. Ce genre de vision peut prendre le nom de vision 

 negative; c'est en effet par une moindre intensite de la portion 

 du champ de la lunette ou existe 1'image de la Lune, que le 

 contour de cette image est aperu. Si 1'image etait plus 

 intense que le reste du champ, la vision serait positive." 

 Arago, Annuaire du Bureau des Longitudes, p. 384. " The 

 Moon is projected partially upon the atmosphere of the Sun. 

 In that portion of the telescope where the image of the Moon 

 is formed, no other light enters except that of the terrestrial 

 atmosphere. The Moon gives no sensible light, and like a 

 screen, it stops all that which comes from beyond and corre- 

 sponds with it. Outside the image, and immediately round 

 its edge, the field is lighted simultaneously by the light of the 

 terrestrial atmosphere, and by that of the solar atmosphere. 

 If we suppose that these two lights collectively, are -fa stronger 

 than the light of the terrestrial atmosphere, the Moon's edge 

 will be directly visible. This kind of vision may be designated 

 a negative vision, for it is in fact by the less intensity of that 

 portion of the field of the telescope in which is the image of the 



