﻿its Polarization and Colour. 277 



suspended particles are large compared with the waves of light, 

 reflection takes place as it would from a portion of the sur- 

 face of a large solid immersed in the fluid, and no conclusion 

 can be drawn either way. But if the diameter of the particles 

 be small compared with the length of a wave of light, it seems 

 plain that the vibrations in a reflected ray cannot be perpendi- 

 cular to the vibrations in the incident ray." This is the only 

 passage that I have met with in which the theory of the reflec- 

 tion of light from very small particles is touched upon. 



If it be assumed, as in the theories of Green and Cauchy of 

 reflection at plane surfaces, that the effect of dense matter is 

 merely to load the ether, it follows rigorously that the direction 

 of vibration cannot be turned through a right angle when light 

 is scattered from small particles. But all we know in the first 

 instance is that the velocity of propagation of luminous waves is 

 less in ordinary transparent matter than in vacuum ; and this 

 may be accounted for as well by a diminished rigidity as by an 

 increased density. In the first case a scattered ray might be 

 composed of vibrations perpendicular to those of the incident 

 beam ; so that the matter is not quite so clear as it would seem 

 from the argument of Professor Stokes. I believe, however, 

 that good reasons may be given for rejecting the view that the 

 difference between media of varying refrangibility is one of rigi- 

 dity. The point is an important one, and I propose to recur to 

 it later. 



The experiments of Professor Tyndall* with precipitated 

 clouds exhibit more clearly than had been done by Briicke the 

 relation between the size of the particles and the nature of the 

 dispersed light. The observation that the polarization is complete 

 perpendicular to the track of the incident light is in itself sufficient 

 to disprove the theory of bubbles. As the particles increase in 

 magnitude, the azure and polarization are gradually lost. During 

 the transition a different and more complicated set of phenomena 

 present themselves, which will furnish a test for the theory when 

 it is extended so as to include the consideration of particles which 

 are no longer very small in comparison with the waves of light. 



All who have written on this subject seem to have taken for 

 granted that the foreign matter in the atmosphere is water or 

 ice. Even Tyndall, who expressly says that any particles, if 

 small enough, will do, still believes in the presence of water-par- 

 ticles. But this view is encumbered with considerable difficulty ; 

 for even if, in virtue of its transparency to radiant heat, the air 

 in the higher regions of our atmosphere is at a very low tempe- 

 rature, it would still be capable of absorbing the very small 

 quantity of water which is sufficient to explain the blue of the 

 * Phil. Mag. vol. xxxvii. p. 385. Phil. Trans. 1870. 



Phil. Mag. S. 4. Vol. 41. No. 273. April 1871. U 



