the Weights of Atoms. 283 



There is very little of polarization in the light from white 

 clouds seen in any direction, or even from a cloudless sky 

 close above the horizon seen at 90° from the sun. This is 

 partly because the particles which give it are not small in 

 comparison with the wave-lengths, and partly because they 

 contribute much to illuminate one another in addition to the 

 sunlight directly incident on them. 



§ 55. For his dynamical foundation, Rayleigh definitely 

 assumed the suspended particles to act as if the ether in their 

 places were denser than undisturbed ether, but otherwise 

 uninfluenced by the matter of the particles themselves. He 

 tacitly assumed throughout that the distance from particle to 

 particle is very great in comparison with the greatest diameter 

 of each particle. He assumed these denser portions of ether to 

 be of the same rigidity as undisturbed ether ; but it is obvious 

 that this last assumption could not largely influence the result, 

 provided the greatest diameter of each particle is very small 

 in comparison with its distance from next neighbour, and with 

 the wave-lengths of the light : and, in fact, I have found from 

 the investigation of §§ 41, 42 of Lecture XIV. for rigid 

 spherical molecules embedded in ether, exactly the same 

 result as Rayleigh's ; which is as follows 



*=-3 ■■(■■ D ^) =8267 H~-i7~ri * ' (1); 



where \ denotes the wave-length of the incident light sup- 

 posed homogeneous ; Tthe volume of each suspended particle ; 

 I) the undisturbed density of the ether ; D' the mean density 

 of the ether within the particle ; n the number of particles 

 per cubic centimetre ; and h the proportionate loss of homo- 

 geneous incident light, due to the scattering in all directions 

 by the suspended particles per centimetre of air traversed. 

 Thus 



1-e-** (2) 



is the loss of light in travelling a distance x (reckoned in 

 centimetres) through ether as disturbed by the suspended 

 particles. 



It is remarkable that D' need not be uniform throughout 

 the particle. It is also remarkable that the shape of the 

 volume T may be anything, provided only its greatest diameter 

 is very small in comparison with \. The formula supposes 

 T (JJ f ■— Z>) the same for all the particles. We shall have to 

 consider cases in which differences of T and !>' for different 



U2 



