SUNLIGHT AND ITS MEASUREMENT L65 



produced in 50 cm. of atmospheric air. Ionization of the air 

 removes some energy from the solar beam and appears to be of 

 direct physiological importance. 22 



Besides the effects so far considered, which occur at rather 

 isolated portions of the spectrum, there is another effect that 

 is common to all radiation but varies in degree from wave 

 length to wave length: the "scattering" of the radiation by 

 the gases of the atmosphere to which is attributed the blue 

 color of the sky. 23 Briicke 24 and Tyndall 25 produced artificial 

 skies in tubes by forming clouds by gaseous chemical reactions. 

 As the particles became larger and larger the color, at first 

 azure blue, assumed a whitish cast indicating that the particles 

 were so large that they reduced the light by regular reflection. 

 Rayleigh 26 came to the conclusion that the blue color is produced 



22 Spoehr, H. A., Variations in respiratory activity in relation to sunlight. 

 Bot. Gaz. 59: 366-386. 1915. 



23 Nichols, E. L., Theories of the color of the sky. Phys. Rev. 26: 497-511. 

 1908. 



24 Briicke, E., Ueber die Farben whelche triibe Medien in auffallenden und 

 ■durchfallenden Licht zeigen. Pogg. Ann. 88: 363-385. 1913. 



25 Tyndall, J. On the blue colour of the sky, the polarization of skylight by 

 ■cloudy matter generally. Proc. Roy. Soc. London 17: 223-233. 1868. 



26 Rayleigh (J. W. Strutt), On the light from the sky, its polarization and 

 colour. Phil. Mag. (4) 41: 107-120; 274-279. 1871. 



Idem. On the scattering of light by small particles. Phil. Mag. (4) 41: 

 447^54. 1871. 



Idem. On the transmission of light through an atmosphere containing small 

 particles in suspension and on the origin of the blue of the sky. Phil. Mag. 



(5) 47: 375-384. 1899. 



Burton, C. V., Scattering and regular reflection of light by gas molecules. 

 Phil. Mag. (6) 29: 625-636. 1915; (6) 30: 87-105. 1915. 



The somewhat extensive controversy regarding the ability of gas molecules 

 to scatter light seems to be settled by recent experiments on the subject, see: 



Strutt, R. J., Scattering of light by dust free air, with artificial reproduction 

 of the blue sky — Preliminary note. Proc. Roy. Soc. London 94A: 453-459. 

 1918. 



Idem. The light scattered by gases; its polarization and intensity. Proc. 

 Roy. Soc. London 95A: 155-176. 1918. 



Wood, R. \\\, The scattering of light by air molecules. Phil. Mag. (6) 36: 

 272-273. 1918. 



Strutt, R. J., The scattering of light by air molecules. Phil. Mag. (6) 36: 

 320-321. 1918. 



Larmor, J., The principle of molecular scattering of radiation. Phil. Mag. 



(6) 37: 161-163. 1919. 



