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, 

 quibus Astronomies pars Optica traditur, 1604, 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 

 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, $ 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 Acadtmie des Sciences, le 5 Aout, 1833, " M. Arago sig- 

 nale la comparaison de 1'intensite lumineuse de la portion de la Lune 

 que les rayons solaires 6clairent directement, avec celle de la partie du 

 me me astre qui re^oit seulement les rayons reflechis par la Terre. II 

 croit d'api'es les experiences qu'il a deja tentees a cet egard, qu'on 

 pourra, avec des instrnmens perfectionnes, saisir dans la lumiere cendrc 

 les differences de 1'eclat plus ou moins nuageux de 1'atmosphere de 

 uotre globe. 11 n'est done pas impossible, malgre tout ce qu'uu pareil 

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

 teorologistes aillent patter dans 1'aspect de la Lune des notions pre- 

 cieuses sur Vtat moyen de diaphanite de 1'atmosphere terrestre, dans les 

 hemispheres qui successivement concourreut 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 \vill be possible to detect in the ashy 

 light indications of the differences in brightness, more or less cloudy, 

 ot 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. 



t Kepler, Paralip. vel Astronomic pars Opticee, 1604, p. 297. 



VOL. IV. G 



