On the Blue Colour of the Sky. 267 



sive sensations. History also records the sensational character 

 of the destruction of Pompeii. If Mr. Scrope's innuendo re- 

 garding the internal fluidity of the earth as " a sensational hypo- 

 thesis " has any value, we should regard the events referred to 

 as highly improbable ; yet they have been as well authenti- 

 cated as the most positive facts in science, and no person has 

 ever expressed the smallest shadow of a doubt as to their oc- 

 currence. 



XXXIV. On the Blue Colour of the Sky. By Arthur Mason 

 Worthington, Trin. Coll. Oxford, M.A., F.R.A.S., Head 

 Master of the Salt Schools, Shipley. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 



IX the series of articles which Mr. Xorman Lockyer is pub- 

 lishing in the columns of ' Xature,' entitled " Physical 

 Science for Artists," he broaches an explanation of certain 

 effects of aerial perspective and sky-colour, which appears to 

 me inconsistent in itself. It is, moreover, entirely at variance 

 with the explanation which I believe to be generally accepted 

 by those who have thought about the matter, and which is 

 stated without any reserve by Professor Helmholtz in the last 

 volume of his popular lectures published in 1876, in a lecture 

 entitled " Optisches iiber Malerei." Mr. Lockyer writes with 

 well-deserved authority ; and it is, I think, for this very 

 reason unfortunate that he should set before professedly un- 

 scientific readers (since he writes for artists) a novel expla- 

 nation of his own, without at least indicating that there is an 

 entirely different and generally accepted explanation of the 

 phenomena of which he is speaking ; for he alludes (p. 156, § 4) 

 to Dr. Tyndall's conclusion as somewhat similar to his own. 



I wish specially to draw attention to the apparent incon- 

 sistency of his explanation of the blue colour of the air so fre- 

 quently seen between the observer and a distant mountain. 



In So. 4 of the series (May 30) he explains that gold is 

 yellow because, out of the white light which enters it, the com- 

 ponents at each end of the spectrum are absorbed in their trans- 

 mission into the mass of metal, and so there remain over only 

 the middle rays, which, if transmitted, appear green, if reflected, 

 yellow, owing to the greater quantity of the light. Clearly 

 he regards the colour of gold as green (or yellow), because 

 the other colours are absorbed. He then quotes from Pro- 

 fessor Stokes's admirable South-Kensington Lecture, and 

 accepts his conclusion, that a poppy is red because out of the 



