the moon's light. 145 



double course from the Earth to the Moon, and from thence 

 to our eye. " Thus, when we have better photometric in- 

 struments at our command, we may be able," as Arago re- 

 marks,* " to read in the Moon the history of the mean con- 

 dition of the diaphaneity of our atmosphere." The first cor- 

 rect explanation of the nature of the ash-colored light of the 

 Moon is ascribed by Kepler (ad Vitellionem Paralipomena, 

 quibits Astro nomicc pars Optica traditicr, 1604, p. 254) to 

 his highly venerated teacher Miistlin, who had made it known 

 in a thesis publicly defended at Tubingen in 1596. Galileo 

 spoke (Sidcreus Nimcius, p. 26) of the reflected terrestrial 

 light as a phenomenon which he had discovered several years 

 previously ; but a century before Kepler and Galileo, the ex- 

 planation of terrestrial light visible to us in the Moon had not 

 escaped the all-embracing genius of Leonardo da Vinci. His 

 long-forgotten manuscripts furnished a proof of this.f 



In the total eclipse of the Moon, the disk very rarely dis- 

 appears entirely ; it did so, according to Kepler's earliest ob- 

 servation, X on the 9th of December, 1601, and more recently, 

 on the 10th of June, 1816; in the latter instance so as not 

 to be visible from London, even by the aid of telescopes. The 

 cause of this rare and extraordinary phenomenon must be a 



* Stance de V Academic des Sciences, le 5 Aoiit, 1833, " M. Arago sig- 

 uale la comparaison de l'intensite lumineuse de la portion de la Luno 

 que les rayons solaires eclairent directemeut, avec celle de la partie du 

 meme astre qui recoit seulement les rayons reflechis par la Terre. II 

 croit d'apres les experiences qu'il a cleja tentees a cet egard, qu'on 

 pourra, avec des instrumens perfectionnes, saisir dans la lumiere cendre'e 

 les differences de l'eclat plus on moins nuageux de l'atmosphere de 

 notre globe. 11 n'est done pas impossible, malgre tout ce qu'un pareil 

 resultat exciterait de surprise au premier coup d'oeil, qu'un jour les me- 

 teorologistes aillent puiser dans l'aspect de la Lune des notions pre- 

 cieuses sur Vtlat moyen de diaphanite de l'atmosphere terrestre, dans les 

 hemispheres qui successivement concourrent a la production de la lu- 

 miere cendree." " M. Arago pointed out the comparison between the 

 luminous intensity of that portion of the Moon which is illuminated di- 

 rectly by the solar rays, and that portion of the same body which re- 

 ceives 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, 

 ol the atmosphere of our globe. It is not, therefore, impossible, not- 

 withstanding 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 diaphaneity of our atmosphere in the hemispheres 

 which successively contribute to the production of the ashy light." 



t Venturi, Essai sur les Ouvrages de Leonard de Vinci, 1797, p. 11. 



X Kepler, Paralip. vel Astronomies pars Optiae, 1604. p. 297. 



Vol. IV.— G 



