AND THE THEORY OF LIGHT. 25 



be necessary to produce any effect. In order therefore to 

 explain the phenomenon of diffused light, we must suppose 

 the atmosphere to be composed of molecules (of a spherical 

 form for instance), each of which presents an image of the 

 sun somewhat in the same manner as an ordinary glass 

 ball. Pure air is blue, because^ according to Newton, 

 the molecules of air have the necessary thickness to reflect 

 blue rays. It is therefore natural that the small images 

 of the sun, reflected by the spherical molecules of the 

 atmosphere, should present a bluish tinge; this colour 

 is not, however, pure blue, but white in which blue predo- 

 minates. When the sky is not perfectly pure and the 

 atmosphere is blended with perceptible vapours, the dif- 

 fused light is mixed with a large proportion of white. As 

 the moon is yellow, the blue of the air assumes somewhat 

 of a greenish tinge at night, or in other words, becomes 

 blended with yellow."* 



40. We are first of all told, on the authority of Newton, 

 that the particles of air are of a size to reflect only blue, 

 and then that the blue is not pure blue, but white in 

 which blue largely predominates. If it be white in which 

 blue largely predominates, the particles of air in any case 

 must be able to reflect other rays than the blue by this 

 admission. Then, again, we are told that the moon is 

 yellow, and that the light of the moon mixing with the 

 blue of the air gives the appearance of green. In this 

 case the air cannot be pure ; there must be vapours in it 

 which reflect yellow, or how could there be a mixture to 

 produce green? Or, if the light of the moon is yellow, 

 from whence comes the blue ? 



41. It is in vain to accumulate instances of inconclusive 

 reasoning in the theory of light, stumbling blocks in the 

 path of science. According to this method of arguing, if the 

 moon is yellow, the air when pure should be black or blue. 



* Yol. iii. p. 88. Bohn. 

 SEE. III. VOL. I. E 



