the Colour of the Sky. 427 



Even Helmholtz, to whom more than to any other we owe 

 our knowledge of this fact, seems to have overlooked its ap- 

 plication to the present question. On page 66 of the Popu- 

 Idre wissenschaftliche Vortrage, Heft 3, where the blueness of 

 the sky is mentioned, he contents himself with Newton's ex- 

 planation, illustrating the relative effects of coarsely and finely 

 divided matter as follows : — 



u The colour of the light reflected by the intervening par- 

 ticles depends largely upon their size. If a splinter of wood 

 float upon the water and we make a little set of wave-rings 

 near it by means of a falling drop of water, the waves will be 

 reflected by the floating splinter as they would be by a stone 

 wall. In the long ocean-swells, however, the little splinter 

 would be tossed to and fro without noticeably impeding the 

 progress of the waves. Now light, as we know, is also the 

 wave-like progressive motion of a medium pervading the uni- 

 verse, and called the aether. The red and green rays have the 

 longest waves, the violet and blue the shortest. Very fine 

 particles, which disturb the homogeneity of the aether, must 

 therefore reflect the latter rays more readily than they do the 

 red and green light. In point of fact, the light reflected by 

 such a medium is bluer the finer its particles, while coarser 

 particles reflect all colours and give a white effect. Such is 

 the blue of the heavens, that is to say of the atmosphere, seen 

 against the blackness of open space. The purer and more 

 transparent the air, the bluer are the skies." 



An attempt to decide how far this illustration really re- 

 presents the process of atmospheric reflexion involves a discus- 

 sion of the mechanical causes of reflexion in general. There 

 are two hypotheses : — 1st. That reflexion and refraction result 

 from the action of the particles of the reflecting body upon 

 the vibrating particles of aether. 2nd. That they arise from 

 the difference between the density of the aether within the 

 body and that within the adjacent medium. Fresnel (CEuvres 

 Completes, tome i., " Memoire sur la Reflexion "), after long 

 making use of the latter theory, rejected it for the first one, 

 which seemed better applicable to the phenomena connected 

 with Newton's rings. 



Assuming such an action between the particles of the body 

 and of the aether, it might be difficult to decide, in the case 

 where the body is exceedingly small, for or against the possi- 

 bility of a selective reflexion of the more refrangible rays. 

 Since FresnePs time this theory has been generally aban- 

 doned for the second one. A strong argument against it may 

 be deduced from the refracting powers of gases. The various 

 gases contain, by AvogadiVs law, an equal number of mole- 



2 G2 



