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XLI. Light Scattering by Air and the Blue Colour of the Sky. 

 By U. W. Wood, For.Mem. M.S.. Professor of Experimental 

 Physics, Johns Hopkins University*. 



r|^HE scattering of light by dust-free air was first observed 

 JL by Cabannes (Comptes Pendus, clx. p. 62), who 

 focussed the image of a quartz-mercury arc at the centre of 

 a dark box lined with black velvet, viewing the scattered 

 radiation through a glass window in the wall of the box on a 

 line formed by the prolongation of the image of the arc. 

 Strutt, working independently and without knowledge of the 

 investigation of Cabannes, observed the same phenomenon 

 and made a very comprehensive study of the relative scat- 

 tering power of different gases, its dependence upon the 

 density of the gas and its state of polarization (Proc. Roy. 

 Soc. vol. xciv. p. 453, vol. xcv. p. 155, 1918). 



The results of certain investigations of atmospheric trans- 

 mission appear to justify Lord Rayleigh's theory that the 

 blue sky is completely accounted for by the scattering of the 

 air molecules themselves, independently of the presence of 

 any foreign matter. 



So far as I am aware, however, no attempts have been 

 made, up to the present time, to compare the scattering 

 exhibited by dust-free air in a tube with the direct light of 

 the sky (as to intensity), or to compare the scattering power 

 of the air near the surface of the earth on a very clear day 

 with the average scattering power of the whole atmosphere. 



Abbot's work on the absorption of light by the atmosphere 

 showed that the loss of intensity due to passage through the 

 lower mile of air above Washington was practically equal 

 to the total loss resulting from passage through the entire 

 atmosphere above the first mile (roughly four miles of homo- 

 geneous atmosphere). This indicates that its scattering 

 power is considerably greater than that of the higher atmo- 

 sphere, where the foreign matter is present in much smaller 

 quantities. There would doubtless be less foreign matter in 

 country locations. 



The present paper deals with the scattering power of the 

 air close to the ground, and the photometric comparison of 

 the intensity of the light scattered by dust-free air, when 

 illuminated by a concentrated beam of sunlight, with the 

 intensity of the blue sky on a very clear day. 



We will commence with the second subject. 



The air was contained in a tube of black fibre 10 cm. in 



* Communicated by the Author. 



