97 



by direct proof, whether the cyanometrical ob- 

 servations are comparable with each other, I 

 have often placed the instrument in the hands 

 of persons, who had not been accustomed to this 

 kind of measurement, and I have observed, that 

 their judgment on the shades of blue toward the 

 horizon and at the zenith never differed more 

 than two degrees. 



The chamois hunters and Swiss herdsmen 

 have at all times been struck with the intense 

 color of the heavenly vault on the summit of the 

 Alps. In the year 1765 Mr. Deluc fixed the 

 attention of naturalists on the phenomenon, the 

 causes of which he developed with equal preci- 

 sion and simplicity. u In the lower part of the 

 atmosphere," says he % " the color of the air is 

 always paler and weakened by the vapors, which 

 at the same time cause a greater dispersion of 

 the light. The air of the plains becomes deeper, 

 when it is more pure ; but it never approaches 

 the vivid and deep tint, which is remarked on the 



t. xxxviii, p. 499. Voyages dans les Alpes, § 2086. Essai 

 sur la Geographic des Plantes, 1807, p. 102. Bouguer ap- 

 pears to have already had the idea of a ^milar instrument, 

 but of more general use. In speaking of the light reflected 

 by the particles of the air, he says, *' We should employ, as 

 a term of comparison, painted tablets of different colors." 

 Traite d'Optique, p. 865. 



* Researches on the Modifications of the Atmosphere, 

 §931. 



