an Atmosphere containing Small Partides in Suspension. 383 



must be supposed to be oriented in all directions indifferently. 

 But the theoretical conclusion that the light diffracted in a 

 direction perpendicular to the primary rays should be com- 

 pletely polarized may well be seriously disturbed. If the 

 view, suggested in the present paper, that a large part of the 

 light from the sky is diffracted from the molecules themselves, 

 be correct, the observed incomplete polarization at 90° from 

 the Sun may be partly due to the molecules behaving rather 

 as elongated bodies with indifferent orientation than as spheres 

 of homogeneous material. 



Again, the suppositions upon which we have proceeded 

 give no account of dispersion. That the refraction of gases 

 increases as the wave-length diminishes is an observed fact ; 

 and it is probable that the relation between refraction and 

 transparency expressed in (14) holds good for each wave- 

 length. If so, the falling off of transparency at the blue end 

 of the spectrum will be even more marked than according to 

 the inverse fourth power of the wave-length. 



An interesting question arises as to whether (14) can be 

 applied to highly compressed gases and to liquids or solids. 

 Since approximately (/jl — 1) is proportional to n, so also is 

 h according to (14). We have no reason to suppose that 

 the purest water is any more transparent than (14) would 

 indicate ; but it is more than doubtful whether the calcula- 

 tions are applicable to such a case, where the fundamental 

 supposition, that the phases are entirely at random, is violated. 

 When the volume occupied by the molecules is no longer 

 very small compared with the whole volume, the fact that 

 two molecules cannot occupy the same space detracts from 

 the random character of the distribution. And when, as in 

 liquids and solids, there is some approach to a regular spacing, 

 the scattered light must be much less than upon a theory of 

 random distribution . 



Hitherto we have considered the case of obstacles small 

 compared to the wave-length. In conclusion it may not be 

 inappropriate to make a few remarks upon the opposite 

 extreme case and to consider briefly the obstruction presented, 

 for example, by a shower of rain, where tbe diameters of the 

 drops are large multiples of the wave-length of light. 



The full solution of the problem presented by spherical 

 drops of water w r ould include the theory of the rainbow, 

 and if practicable at all would be a very complicated matter. 

 But so far as the direct light is concerned, it would seem to 

 make little difference whether we have to do with a spherical 

 refracting drop, or with an opaque disk of the same diameter. 



2D2 



