Molecular Scattering of Radiation. 163 



them will thus vary within a range of 10" 6 of itself. If the 

 phases of the scattered radiations are correlated at first, after 

 traversing 10 6 wave-lengths or 50 cm. they will have become 

 fortuitous, and the energy-effects thus additive. 



This consideration, if justified, would find the source of 

 Lord Rayleigh's principle in the uncoordinated thermal 

 motions of the molecules. And as in a crystal thermal 

 vibrations are contemplated about the mean positions of the 

 molecules in the space-lattice, there ought to be some degree 

 of scattering in traversing a crystal. 



This way of envisaging the matter contemplates a scattered 

 beam of radiation of wave-length slightly indefinite, on 

 Doppler principles, in which therefore the phases of the 

 constituent elements become fortuitous after travelling a 

 substantial though not very great distance. Other influences 

 of radiation on the gas which are also adjusted over a 

 considerable range in distance would be implied : the mole- 

 cular effects connected with pressure of radiation would be 

 expected to belong to this class. 



Some confirmation from another point of view seems 

 desirable. The scattered radiation, forming a spectral band 

 of some slight breadth, may be considered as resolved into 

 more homogeneous constituents. The radiation in one of 

 them has been scattered at each instant by molecules whose 

 thermal velocities agree within close limits as regards both 

 direction and magnitude : these molecules constitute a sparsely 

 distributed group whose distances apart can be of the order 

 of the wave-length, so that the condition necessary for 

 fortuitous phases in the scattered disturbances is satisfied. 



The interesting remark is made by Lord Rayleigh (p. 445) 

 that the radiation scattered nearly in the direction of the 

 primary rays is specially favoured, in that all its components 

 nearly agree in phase. The question arises whether this 

 would not make a clear sky very much brighter within a 

 few degrees of the sun than at some distance away from it. 

 Here also the slight variety of wave-lengths arising from 

 the thermal motions of the scattering molecules would seem 

 to reduce or perhaps nearly remove such disparity. 

 Cambridge, Dec. 16, 1918. 



