282 Lord Kelvin on 



11 of the particles be small compared with the length of a wave 

 u of light, it seems plain that the vibrations in a reflected ray 

 ." cannot be perpendicular to the vibrations in the incident 

 " ray" ; which implies that the light scattered in directions 

 perpendicular to the exciting incident ray has everywhere its 

 vibrations perpendicular to the plane of the incident ray and 

 the scattered ray ; provided the diameter of the molecule 

 which causes the scattering is very small in comparison with 

 the wave-length of the light. In conversation Stokes told me 

 of this conclusion, and explained to me with perfect clearness 

 and completeness its dynamical foundation ; and applied it to 

 explain the polarization of the light of a cloudless sky, viewed 

 in a direction at right angles to the direction of the sun. 

 But he did not tell me (though I have no doubt he knew it 

 himself) why the light of the cloudless sky seen in any direc- 

 tion is blue, or I should certainly have remembered it. 



§ 53. Eayleigh explained this thoroughly in his first paper 

 (1871), and gave what is now known as Rayleigh's law of the 

 blue sky ; which is, that, provided the diameters of the 

 suspended particles are small in comparison with the wave- 

 lengths, the proportions of scattered light to incident light for 

 different wave-lengths are inversely as the fourth powers of 

 the wave-lenoths. Thus, while the scattered light has 

 the same colour as the incident light wdien homogeneous, the 

 proportion of scattered light to incident light is seven times 

 as great for the violet as for the red of the visible spectrum ; 

 which explains the intensely blue or violet colour of the- 

 clearest blue sky. 



§ 54. The dynamical theory shows that the part of the light 

 of the blue sky, looked at in a direction perpendicular to the 

 direction of the sun, which is due to sunlight incident on a 

 single particle of diameter very small in comparison with 

 the wave-lengths of the illuminating light, consists of 

 vibrations perpendicular to the plane of these two directions ^ 

 that is to say, is completely polarized in the plane through the 

 sun. In his 1871 paper * Eayleigh pointed out that each 

 particle is illuminated, not only by the direct light of the sun, 

 but also by light scattered from other particles, and by earth- 

 shine, and partly also by suspended particles of dimensions 

 not small in comparison with the wave-lengths of the actual 

 light ; and he thus explained the observed fact that the 

 polarization of even the clearest blue sky at 90° from the sun 

 is not absolutely complete, though it is very nearly so.. 



* Collected Papers, vol. i. p. 94. 



