268 Mr. A. M. Worthington on the 



white light that enters it and emerges from it to be reflected, 

 only the red survives absorption ; the rest is absorbed in the 

 interior of the leaf. 



So far, beyond his own explanation of the molecular group- 

 ings which cause this selective absorption, Mr. Lockyer has 

 advanced nothing new ; he has dealt only with what are called 

 " natural bodies." But in No. 6 of June 6th, he apparently 

 applies the explanation of the colours of natural bodies to the 

 colour of the air between the observer and a distant hill ; and 

 here I cannot follow him. 



He says, page 155, § 2 : — 



"We are in the presence of aqueous vapour competent to 

 be set in vibration by blue light, and because it vibrates in 

 this way it appears blue." And in the next paragraph: — 



" If the stratum of aqueous vapour had had a background 

 of bright sky, it would have absorbed the blue light of that 

 sky. By virtue of the principles which I have stated, the sky 

 would have appeared red in consequence of the abstraction of 

 blue light." 



Mr. Lockyer here clearly regards the molecules as abstract- 

 ing the blue rays ; as being set in synchronous vibration by 

 them, and therefore sending blue light to the eye. He can 

 hardly mean that the phenomenon is one of selective absorp- 

 tion and emission, for no vapour emits visible light until its 

 temperature is very high : indeed there seems no doubt that 

 he considers the air to appear blue by reflected light ; for he 

 says, speaking of similar water- vapour, on page 156, §§2 and 

 3: — " Let us consider, then, the action of those molecules which 

 absorb the blue light. 



" Now, since these molecules absorb blue light, we know that 

 they will reflect blue light, and, practically speaking, nothing 

 else. Here, then, we have the cause for the blue colour of 

 the sky." 



Why ! Mr. Lockyer has himself explained that gold is 

 yellow beceuse it absorbs the red and blue and reflects the 

 yellow light ; that a poppy is red because it absorbs all but 

 the red ; and now he says that water-vapour is blue because 

 it absorbs blue. 



Apart from the inconsistency of this explanation, I do not 

 think that, as a matter of fact, Professor Stokes's explanation 

 of the colours of " natural bodies " can be applied to the ap- 

 pearance of a mass of air in which particles of aqueous vapour 

 are suspended. The following is what Professor Helmholtz 

 says of the same phenomenon, translated from the lecture I 

 have mentioned. On page 65, § 3, he says: — " Under the head 

 of aerial perspective we understand the optical effect of the 



