422 Professor Forbes on the Colours of the Atmosphere. 



air he seems to have believed to be devoid of colour, and the ' 



reflective particles to consist of vapour foreign to it. 



The idea of Mariotte of the inherent quality of the sky to 

 reflect blue light, was next prominently stated by Bouguer, 

 who further put it in so palpable a form as to have been ge- 

 nerally quoted since as a complete explanation of aerial co- 

 lours*. He observes, that as red light penetrates further than 

 blue (the reason is not mentioned), the latter is wholly reflected, 

 whilst the former reaches the eye ; and this theory was further 

 improved by later writers, by ascribing superior momentum to 

 the red rays, and inferior to the more refrangible ones. Smith, 

 the author of the System of Optics, states the same view, but 

 with greater clearness. " The blue colour of a clear sky," 

 he says, " shows manifestly that the blue-making rays are 

 more copiously reflected from pure air than those of any 

 other colour ; consequently they are less copiously transmitted 

 through it among the rest that come from the sun, and so , 



much the less as the tract of air through which they pass is 1 



the longer. Hence the common colour of the sun and moon ' 



is whitest in the meridian, and grows gradually more inclined 

 to diluted yellow, orange, and red, as they descend lower; 

 that is, as the rays are transmitted through a longer tract of 

 airf;" and so he explains the colour of the moon in eclipses 

 by the altered light refracted by the earth's atmosphere. 



Next, Euler (1762) maintained the same opinion as to the 

 blueness of the sky. " It is more probable," he says, " that 

 all the particles of the air should have a faintly bluish cast, but 

 so very faint as to be imperceptible, until presented in a pro- 

 digious mass, such as the whole extent of the atmosphere, 

 than that this colour is to be ascribed to vapours floating in 

 the air, which do not pertain to it. In fact, the purer the air 

 is, and the more purged from exhalation, the brighter is the 

 lustre of heaven's azure, which is a sufficient proof that we 

 must look for the reason of it iii the nature of the proper par- 

 ticles of the air X" 



The Abbe NoUet (1764) attributes the blue colour of the 

 sky to its reflecting those rays ; but, strangely enough, he sup- 

 poses, that, in order to convey that tint to the eye, they must 

 previously have come to the earth, been reflected by it, and 

 stopped in their second transit through the atmosphere. The 



* Traitc d'Optique, p. 3G5-368. He likewise explains the coloured 

 shadows noticed by Buffon. 



+ Smith's Optics, vol. ii. Remarks, 378. 

 X Euler's Letters (translation), ii. 507. 



