and the Blue Colour of the Sky. 425 



width to the circumference through which it moves, in this 

 case 1/4867. 



As we now know from Strutt's experiments, the light 

 scattered by dust-free air is almost completely polarized. 

 The light of the sky exhibits, however, not much over 

 €0 per cent, of polarization in a direction perpendicular to 

 the exciting rays. It seems reasonable to infer from this 

 that about 40 per cent, of its light is due to secondary 

 scattering (scattering of light coming from the rest of the 

 sky and the earth together with a certain amount scattered 

 by the larger particles forming the haze found at lower levels). 

 This means that the sky as observed in the experiment had 

 an intensity about 1*7 times as great as would be shown by a 

 column of air five miles in depth illuminated only bythe rays 

 of the sun, and viewed end-on, which is really what is to be 

 compared w T ith the tube illumination. We have therefore 

 effected a reduction of intensity with the rotating disk 1*7 

 times as great as would have been required if the conditions 

 were as just specified. 



Applying this correction alters our ratio of 1/4867 to 1/2860. 

 This is to be compared with the ratio calculated for the 4 mm. 

 of air illuminated in the tube and the five miles of air forming 

 the sky. Sunlight at sea-level according to Abbot's tables 

 has a value for the blue-green portion of the spectrum of 

 about 50 per cent, of its value in space, when the sun is at a 

 distance of 60° from the zenith. This has been increased 

 1400 times by the lens, and we can therefore represent the 

 scattered illumination in the tube (on an arbitrary scale), 

 if we call the intensity of sunlight in space unity, by 

 1400x4x1/2 = 2800. 



We must now compute the scattered intensity which w r e 

 should expect from the atmosphere on the same arbitrary 

 scale. Since we are observing a point 30° from the zenith, 

 the effective depth through which we are observing is about 

 1*2 times the zenith depth, or six miles of homogeneous 

 atmosphere. If we consider the sky as due to the illumination 

 of this depth of air by sunlight of its full intensity in space 

 (unity), the illumination will be represented by the number 

 of millimetres in six miles, or 9,600,000, while our tube 

 illumination was 2800. 



The ratio of these two calculated numbers is 1/3430, while 

 the corrected ratio measured with the photometer was 

 1/2860. 



The agreement is remarkably good considering the 

 enormous difference between the two intensities compared 

 experimentally, and the uncertainty about just what values 



