142 COSMOS. 



feeble than the sunlight which is reflected by a white cloud 

 in the daytime. When, in determining geographical longi- 

 tudes, it is often necessary to take the distance of the Moon 

 from the Sun, it is not unfrequently difficult to distinguish the 

 Moon between the more intensely luminous masses of cloud. 

 Upon mountain-heights, which lie between 12,791 and 17,057 

 feet above the level of the sea, and where, in the clear mount- 

 ain air, only feathery cirri are to be seen in the sky, I found 

 the detection of the Moon's disk was much more easy, be- 

 cause the cirrus reflects less sunlight on account of its loose 

 texture, and the moonhght is less weakened by its passage 

 through the rarer strata of air. The relative degree of in- 

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

 new investigation, as Bouguer's universally received determ- 

 ination, -go oVo 0' differs so widely from the certainly less prob- 

 able one of Wollaston, -g ooVo o •* 



The yellow moonlight appears white by day, because the 

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

 plementary color to yellow. t According to the numerous ob- 

 servations which Arago made with his polariscope, the moon- 

 light contains polarized light ; it is most perceptible during 

 the first quarter and in the gray spots of the Moon's surface ; 

 for example, in the great, dark, sometimes rather greenish ele- 

 vated plains, the so-called Mare Crhium. Such elevated 

 plains are generally intersected by metallic veins, in whose 

 polyhedric figure the surfaces are inclined at that angle 

 which is necessary for the polarization of the reflected sun- 

 light. 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 Arh- 

 tarchus, upon which it has been frequently erroneously sup- 

 posed 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 observ* 



* Cosmos, vol. iii., p. 95, and note t. 



i " La lumiere de la Lime est jaune, tandis que celle de V§nus est 

 blanche. Pendant le jour la Lune parait blanche, parcequ'^ la lumiere 

 du disque lunaire se mele la lumiere bleue de cette partie de I'atmo- 

 sphere que la lumiere jaune de la Lune traverse." — Arago, in Handschr. 

 of 1847. " 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 thai 

 part of the atmosphere which the yellow light of the Moon traverses, 

 mixes with the light of the lunar disk." 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. 208, note *.) 



