482 COSMOS. 



" to read in the Moon the history of the mean condition of 

 the diaphanity of our atmosphere." The first correct expla- 

 nation of the nature of the ash-coloured light of the Moon is 

 ascribed by Kepler (ad Vitellionem Paralipomena, quibus 

 Astronomice pars optica traditur, 1 604, p. 254) to his highly 

 venerated teacher Mastlin, who had made it known in a 

 thesis publicly defended at Tubingen in 1596. Galileo spoke 

 (Sidereus Nuncius, p. 26) of the reflected terrestrial light, 

 as a phenomenon which he had discovered several years 



" M. Arago signale la comparaison de l'intensite lumineuse 

 de la portion de la Lune que les rayons solaires eclairent 

 directement, avec celle de la partie du meme astre qui 

 regoit seulement les rayons reflechis par la Terre. II croit 

 d'apres les experiences qu'il a deja tentees a cet egard, 

 qu'on pourra, avec des instrumens perfectionnes, saisir dans 

 la lumiere cendree les differences de 1' eclat plus ou moins 

 nuageux de 1' atmosphere de notre globe. II n'est done pas 

 impossible, malgre tout ce qu'un pareil resultat exciterait de 

 surprise au premier coup d'ceil, qu'un jour les meteorologistes 

 aillent puiser dans 1'aspect de la Lune des notions precieuses 

 sur letat moyen de diaphanite de Tatmosphere terrestre, dans 

 les hemispheres qui successivement concourrent a la produc- 

 tion de la lumiere cendree." "M. Arago pointed out the 

 comparison between the luminous intensity of that portion of 

 the Moon which is illuminated directly by the solar rays, 

 and that portion of the same body which receives only the 

 rays reflected by the Earth. After the experiments which 

 he has already made in reference to this subject, he is of 

 opinion that with improved instruments it will be possible to 

 detect in the ashy light indications of the differences in 

 brightness, more or less cloudy, of the atmosphere of our 

 globe. It is not therefore impossible, notwithstanding the 

 surprise which such a result may excite on the first view, that 

 one day meteorologists will derive valuable ideas as to the 

 mean state of the diaphanity of our atmosphere in the hemis- 

 pheres which successively contribute to the production of the 

 ashy light. " 



