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almost as few blue rays as the dense atmosphere 

 of the plains enlightened by the feeble light of 

 the Moon. From these considerations it follows, 

 that we ought not to say, with Mr. de Saussure 

 and other naturalists, who have recently treated 

 on this subject, that the intensity of the blue is 

 greater on the summit of the Alps than in the 

 plains ; the color of the sky is only deeper, less 

 mixed with white. 



If we direct the cyanometer toward the parts 

 of the sky very near the Sun, the instrument in- 

 dicates, near the zenith, as faint tints as those 

 which are observed near the horizon. The causes 

 of this paleness are very different. Near the Sun 

 too intense a light fatigues our organs ; and the 

 eye, dazzled by the quantity of white rays it re- 

 ceives at once, becomes almost insensible to the 

 impression of the blue rays. At the horizon, on 

 the contrary, it is not the intensity of the aerial 

 light, that renders the azure tint of the sky pale ; 

 before sunset this phenomenon is produced by 

 the white light, reflected by the vapors condens- 

 ed near the surface of the Earth. 



Bouguer has made the curious observation, 

 that, when the Sun is fifteen or sixteen degrees 

 high, there are, on a line parallel to the horizon, 

 two parts of the sky, from 110 to 120 degrees 

 distant from the luminary, where the intensity 

 is at it's minimum ; while we observe the maxi- 

 mum in a point diametrically opposite to the 



