426 Dr, E. L. Nichols's new Explanation of 



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 we find it 

 is by experience." 



This opinion, after nearly two hundred years, has been veri- 

 fied by Tyndall's * well-known experiment. Professor Tyn- 

 dall in his memoir makes no reference to Newton's remark- 

 able foreshadowing of his result, but comes independently to 

 the conclusion that with his tubeful of finely divided matter, 

 which, while its particles were still invisible under the micro- 

 scope, appeared like a faint blue cloud, he has solved the mys- 

 tery of the blueness of the heavens. 



Several years before the publication of Prof. Tyndall's 

 paper, Prof. Clausius f , of Bonn, starting with Newton's 

 idea that the colour of the skies is the blue of the first order, 

 investigated the conditions necessary to the production of such 

 a colour. He shows that the reflecting medium cannot con- 

 sist of minute solid drops, as he thinks Newton to have ima- 

 gined, but must be composed of bubbles, the films of which 

 do not exceed in thickness a quarter of the wave-length of 

 violet light. Well aware, however, that in our atmosphere 

 the particles of vapour assume all sizes up to that of the ordi- 

 nary rain-drop, he goes on to prove that really blue skies 

 occur under conditions of atmosphere admitting of finely 

 divided vapour only, and that changes of pressure and tem- 

 perature such as conduce to the formation of larger particles 

 must at the same time produce in far greater numbers bubbles 

 of the exceedingly small sizes necessary to the blue of the first 

 order ; so that the general tint of the sky will pass into white 

 rather than into any spectral colour. 



How is Newton's opinion, supported by Prof. Tyndall's un- 

 doubted experimental evidence, to be reconciled with Clau- 

 sius's contradictory result? The last-named physicist has 

 shown conclusively that the light reflected by even the smallest 

 particles of matter cannot be blue unless these particles are of 

 vesicular structure. Tyndall has demonstrated the fact that 

 such light is blue. It is the object of this paper to show the 

 bearing upon this subject of a certain well-known fact — the 

 dependence of the relative sensitiveness of the eye for different 

 colours upon the intensity of the ray. It renders clear the 

 nature of the above discrepancy, and affords foundation for a 

 new hypothesis of the colour of the sky. 



* Tyndall, ' Fragments of Science/ " On the Structure and Colour of 

 the Skies." 



t Clausing, PoggendorfFs Annalen, lxxvi. 



