PORTION OF THE COSMOS. THE SOLAR DOMAIN. 355 



with a comet -finder having a very light field, the remarkable 

 observation of a change of the grey or ashy light of the 

 Moon into an olive-green colour, verging somewhat towards 

 yellow. The moon, which then stood vertically over the 

 Atlantic Ocean, received on its nocturnal side the green 

 earthlight sent to it in a cloudless sky from the forest- covered 

 regions of South America ( 567 ). 



The meteorological state of our atmosphere modifies the 

 intensity of this earthlight, which has to traverse the double 

 path from the Earth to the Moon, and from the Moon again 

 to our eyes. " Thus," as Arago has remarked ( 568 ), " when 

 at some future day better photometric instruments shall be 

 employed, we shall be able to read, as it were, in the Moon the 

 mean state of transparency of our atmosphere." The first 

 correct explanation of the nature of the ashy light of the 

 Moon was given in a letter written by Kepler (ad Yitellionem 

 Paralipomena quibus Astronomiee pars optica traditur, 1604, 

 p. 254) to his highly venerated former instructor Mastlin, 

 and put forward by the latter in 1596, on the occasion of 

 the thesis publicly defended at Tubingen. Galileo (in the 

 Sidereus Nuncius, p. 26) spoke of the reflected earthlight 

 as of a thing which he had himself discovered several years 

 before ; but in reality, a hundred years prior to Kepler and 

 Galileo, the true explanation of the earthlight visible to us 

 on the Moon had not escaped the all-embracing genius of 

 Leonardo di Yinci. His long- forgotten manuscripts have 

 afforded the proof of this ( 56 9). 



In total lunar eclipses it happens in some exceedingly 

 rare cases that the Moon disappears wholly : it did so, ac- 

 cording to Kepler's earliest observation ( 57 ), on the 9th of 



