452 SCIENCE PROGRESS 



subjected to a certain absorption causing a relative diminution 

 in the intensity of the blue light or a relative increase of the red. 

 This absorption is very much increased by the presence of 

 condensed water vapour or mist in the atmosphere. The yellow 

 or red colour of the sun when near the horizon, and the colour- 

 ing of clouds or mountain peaks at sunset, is clearly explained 

 by this influence. The thickness of the atmospheric layer 

 traversed by the rays at this time is a maximum. It is obvious 

 from a simple geometrical consideration that light proceeding 

 from the sun when on the horizon, and more especially when 

 below the horizon, passes through a relatively far greater thick- 

 ness of lower atmosphere in comparison to upper, than when the 

 sun is at the zenith. The absorbing influence of suspended 

 matter present in the lower atmosphere is thus the predominant 

 factor in determining the light from the setting sun. 



If scattering of light according to Rayleigh's theory were the 

 sole influence at work here, it would be expected that the sun 

 viewed on the horizon would be of an invariable colour. How- 

 ever, observation shows that the nature of this light is very 

 variable, showing that the elements in the atmosphere which 

 filter out the blue rays of the transmitted light are not constant. 

 Light transmitted from the setting sun through a clear sky 

 is frequently not so red as would be calculated from the theory 

 of scattering. Spectro-photometric measurements have been 

 made of light from the sun passing through a cloudless sky 

 when viewed below the horizon from a high mountain. The 

 nature of the light transmitted under these conditions does not 

 generally conform to Rayleigh's law of molecular scattering, but 

 indicates the presence of other factors of absorption. 



Cases have indeed been placed on record where, in the 

 tropics, the air was exceptionally dry, the light transmitted from 

 the sun on first appearing above the horizon was green. This, 

 if authentic, would definitely establish the presence of a true 

 absorption colour of air. 



The Polarisation of Light from the Sky. — According to 

 Rayleigh's theory, if the whole of the light proceeding from the 

 sky is the result of scattering by molecules and particles which 

 are small compared with the wave-length of light, then light 

 which proceeds from that portion of the sky which is viewed 

 in a direction at right angles to the direction of the sun's rays 

 should be completely polarised. It might conversely be 



