478 COSMOS. 



tensity of tlie Sun's light to that of the full Moon deserves a 

 new investigation, as Bouguer's universally received deter- 

 mination -30 oVo^> differs so widely from the certainly less 

 probable one of Wollaston, -g^oVo o^ 18 



The yellow moonlight appears white by day, because the 

 blue strata of air through which we see it present the com- 

 plementary colour to yellow. 19 According to the numerous 

 observations which Arago made with his polariscope, the 

 moonlight contains polarized light; it is most perceptible 

 during the first quarter and in the grey spots of the Moon's 

 surface ; for example, in the great, dark, sometimes rather 

 greenish walled plains, the so-called Mare Crisium. Such 

 walled plains are generally intersected by mountain veins, 

 in whose polyhedric figure the surfaces are inclined at 

 that angle which is necessary for the polarization of the 

 reflected sunlight. The dark tint of the surrounding space 

 appears in addition to make the phenomenon still more obvious. 

 With regard to the luminous central mountain of the group 

 Aristarchus, upon which it has been frequently erroneously 

 supposed that volcanic action has been seen, it did not present 

 any greater polarization of light than other parts of the 

 Moon. In the full Moon no admixture of polarized light was 

 observable ; but during a total eclipse of the Moon (31st of 



18 Cosmos, vol. iii. p. 126 and note. 



19 " La lumiere de la Lune est jaune, tandis que celle de 

 Venus est blanche. Pendant le jour la Lune parait blanche, 

 parcequ'a la lumiere du disque lunaire se mele la lumiere 

 bleue de cette partie de 1'atmosphere que la lumiere jaune 

 de la Lune traverse." (Arago, in Handschr. of 1847), kt The 

 light of the Moon is yellow, while that of Venus is white. 

 The Moon appears white during the day, because the blue 

 light of that part of the atmosphere which the yellow light of 

 the Moon traverses, mixes with the light of the lunar disc." 

 The most refrangible rays of the spectrum, from blue to violet, 

 unite with the less refrangible, from red to green, to form 

 white. (Cosmos, vol. iii. p. 282, note 19.) 



