﻿108 The Hon. J. W. Strutt on the Light from the Sky, 



refraction are explained in the wave theory is sufficient to show 

 that they have no application unless the surface of the disturbing 

 body is larger than many square wave-lengths ; whereas the par- 

 ticles to which the sky is supposed to owe its illumination must 

 be smaller than the wave-length, or else the explanation of the 

 colour breaks down. The idea of polarization by reflection is 

 therefore quite out of place; and that "the law of Brewster 

 does not apply to matter in this condition " (of extreme fineness) 

 is only what might have been inferred from the principles of the 

 wave theory. 



Nor is there any difficulty in foreseeing what, according to the 

 wave theory, the direction of polarization ought to be. Conceive 

 a beam of plane-polarized light to move among a number of par- 

 ticles, all small compared with any of the wave-lengths. The 

 foreign matter, if optically denser than air, may be supposed to 

 load the aether so as to increase its inertia without altering its 

 resistance to distortion, provided that we agree to neglect effects 

 analogous to chromatic dispersion. If the particles were away, 

 the wave would pass on unbroken and no light would be emitted 

 laterally. Even with the particles retarding the motion of the 

 aether, the same will be true if, to counterbalance the increased 

 inertia, suitable forces are caused to act on the aether at all 

 points where the inertia is altered. These forces have the same 

 period and direction as the undisturbed luminous vibrations them- 

 selves. The light actually emitted laterally is thus the same as 

 would be caused by forces exactly the opposite of these acting on 

 the medium otherwise free from disturbance ; and it only remains 

 to see what the effect of such forces would be. 



On account of the smallness of the particles, the forces acting 

 throughout the volume of any one are all of the same intensity 

 and direction, and may be considered as a whole. The determi- 

 nation of the motion in the aether, due to the action of a periodic 

 force at a given point, requires, of course, the aid of mathema- 

 tical analysis ; but very simple considerations will lead us to a 

 conclusion on the particular point now under discussion. In 

 the first place there is a complete symmetry round the direction 

 of the force. The disturbance, consisting of transverse vibra- 

 tions, is propagated outwards in all directions from the centre ; 

 and in consequence of the symmetry, the direction of vibration 

 in any ray lies in the plane containing the ray and the axis; 

 that is to say, the direction of vibration in the scattered or 

 diffracted ray makes w T ith the direction of vibration in the in- 

 cident or primary ray the least possible angle. The sym- 

 metry also requires that the intensity of the scattered light 

 should vanish for the ray which would be propagated along the 

 axis; for there is nothing to distinguish one direction trans- 



