THE MOON S LIGHT. 483 



previously; but a century before Kepler and Galileo, the 

 explanation 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. 25 



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

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

 observation, 26 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 phe- 

 nomenon must be a peculiar and not sufficiently investigated 

 diaphanic condition of individual strata of our atmosphere. 

 Hevelius states distinctly that during a total eclipse, on the 

 25th of April, 1642, the sky was covered with brilliant stars, 

 the atmosphere perfectly clear and yet with the different 

 magnifying powers which he employed, not a vestige of the 

 Moon could be seen. In other cases, likewise very rare, 

 only separate parts of the Moon are feebly visible. During 

 a total eclipse, the disc generally appears red ; and, indeed, 

 in all degrees of intensity of colour, even passing, when the 

 Moon is far distant from the Earth, into a fiery and glowing 

 red. While lying at anchor off the island of Baru, not far 

 from Carthagena de Indias, half a century ago (29th of 

 March, 1801), I observed a total eclipse, and was extremely 

 struck with the greater luminous intensity of the Moon's disc 

 under a tropical sky than in my native north. 27 The whole 



25 Venturi, Essai sur Ics Ouvrages de Leonard de Vinci 

 1797, p. 11. 



88 Kepler, Paralip. vcl Astronomies pars optica, 1604, 

 p. 297. 



27 " On congoit que la vivacite de la lumiere rouge ne 

 depend par uniquement de Tetatde 1'atmosphere, qui refracte, 

 plus ou moins affaiblis, les rayons solaires, en les enflechissant 

 dans le cone d'oinbre, mais qu'elle est modifiee surtout par la 



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