168 HOWARD E. PULLING 



medium in which it is immersed (in the case of dust, clouds, 

 etc., this immersing medium is air), for absorbing substances 

 have a high refractive index on the red side of the absorption 

 band and a low refractive index on the blue side so that a spec- 

 trum of the reflected light will be brightest on the red side of 

 the absorption band. If the small particles are, however, 

 large with respect to the wave length of light the intensity of 

 the reflected or scattered light increases with increasing wave 

 length and in addition varies as the square of the diameter of 

 the particles. Thus the transition from extremely small par- 

 ticles that scatter the light inversely as the fourth power of 

 the wave length, to those of microscopic size affords an enormous 

 range of possibilities. 31 This last statement concerning the 

 influence of the size of particles and the wave length of light 

 may be put into one algebraic expression that indicates the 

 necessary existence of some wave length of the radiation (not 

 necessarily in the visible spectrum) for which the scattered 

 radiation has the greatest intensity. There is also to be taken 

 into account, in the case of radiation that is transmitted through 

 a body, the relative dispersions of this body and the medium 

 in which it is immersed. 32 



From this very brief and fragmentary statement it is obvious 

 that, from the point of view of prediction, little can be said 

 concerning the total effect upon radiation of the atmosphere 

 at any given time. This is not only true because our knowledge 

 of the precise conditions make the inaccuracy of such state- 

 ments inevitable, but also because of the difficulty of drawing 

 quantitative inferences (and in the case of color, predictive 

 statements must necessarily be quantitative) from the very 

 complex equations that would be necessary to include all the con- 

 ditions encountered. However, it may be found that the complex 

 conditions in nature actually exhibit relatively simple indicative 



31 Mie, G., Beitrage zur Optik triiber Medien, speziell Kolloidaler Metal- 

 losungen. Ann. Physik (4) 25: 377-445. 1908. 



32 Wood's Optics, cited in note 1. 



Christiansen, C, Untersuchungen iiber die optischen eigenschaften von 

 fein vertheilten Korpern. Ann. Physik 23: 298-306. 1884. 

 Garnett, cited in note 29. 



