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Article XII. 



On the Blue Colour of the Sky and the Morning and Evening 

 Red. By R. Clausius. 



[From PoggendorfF's Annalen, vol. Ixxvi. p. 188.] 



It is remarked, in the course of the foregoing paper, that the 

 blue colour of the sky and the morning and evening red stand 

 in close connexion with the reflexion of light in the atmosphere. 

 Among many conjectures on this subject, the assumption that the 

 reflexion takes place on minute bladders of water is often stated 

 to be that which explains these phaenomena most completely. 

 For example, Newton* says, — "The blue of the first order, 

 though very faint and little, may possibly be the colour of some 

 substances ; and particularly the azure colour of the skies seems 

 to be of this order. For all vapours, when they begin to con- 

 dense and coalesce into small parcels, become first of that big- 

 ness, whereby such an azure must be reflected, before they can 

 constitute clouds of other colours. And so, this being the first 

 colour which vapours begin to reflect, it ought to be the colour 

 of the finest and most transparent skies, in which vapours are 

 not arrived to that grossness requisite to reflect other colours, 

 as w^e find it is by experienced^ 



Newton does not enter further into the matter, and it appears 

 to me doubtful whether he had a clear view of the subject. 

 When here, and still more plainly in another place fj he says 

 that water particles, when they increase in size, must produce 

 clouds of different colours, in my opinion he is in error, as I 

 shall show further on. It seems also as if by water particles he 

 meant solid spheres instead of hollow vesicles, which supposi- 

 tion, according to the previous discussion, is not admissible. 



After Newton, many investigators have given in their ad- 

 herence to the supposition of vapour vesicles ; but I find the 

 theory nowhere mathematically developed, and it is by no means 



* Optics, vol. II. p. iii. prop. 7. f Jbid., vol. II. p. iii. prop. 5. 



