26 ME. J. SMITH ON THE ORIGIN OF COLOUR 



If there are blue rays in the light by which the atmosphere 

 is illuminated, then it should be blue on this theory ; if 

 there are no blue rays in the yellow of the moon then the 

 air should be black, as it can only reflect blue rays. But 

 the air has a greenish tinge by night on the admission of 

 Arago. My idea is that this compels him to resort to 

 another supposition, namely, that the vapours of the air 

 must necessarily reflect yellow, and as the air itself reflects 

 only the blue, the blending of the two makes green. Why 

 not at once say that the vapours reflect green, for green is 

 a primary colour on Arago's theory of light. But this is 

 merely an instance, among many which can be adduced, 

 of the inconclusiveness of such reasoning founded on the 

 proportioning of the molecules of air to the molecules of 

 light. 



42. I have taken this passage from Cosmos, as it is one 

 with which it appears Humboldt was satisfied, to show 

 how difficult it is to account for the colour of the air on 

 any principles of interpretation yet known, for no one 

 who seriously considers the argument can acquiesce in its 

 validity ; and I attach more importance to it, as I am 

 convinced that if it can be shown that blue or purple can 

 be produced artificially by merely reducing the motion 

 of a white ray, and by the introduction of darkness or 

 shadow, then the argument for the formation of the other 

 colours must also yield to the same principles of reasoning. 



43. There is a similar explanation of the colour of the 

 iair by the author of the article " Optics" in the Library of 

 Useful Knowledge, which, as it is expressed in very beauti- 

 ful and clear language, I shall add to the above from 

 Cosmos. '^ We have already seen that the red rays pene- 

 trate through the atmosphere, while the blue rays, less 

 able to surmount the resistance which they meet, are 

 reflected or absorbed in their passage. It is to this cause 

 that we must ascribe the blue colour of the sky and the 



