24 KIMBALL: NORMAL* ATMOSPHERIC TRANSPARENCY 



During the early part of 1913, it was noticed that after sunset, 

 as the lower atmosphere became less and less illuminated, the 

 skylight polarization increased rapidly, sometimes by as much 

 as 50 per cent, and to nearly the degree of polarization under 

 normal conditions. This increase became noticeably less after 

 the middle of August, 1913.^ 



The solar and the antisolar distances of the neutral points of Bab- 

 inet and Arago, respectively, have varied in accordance with 

 the variations in skylight polarization. This was to be expected, 

 since these distances depend upon the relative intensities of the 

 vertically and the horizontally polarized components of sky- 

 light. 



Table 4 will serve to illustrate the movements of both the neu- 

 tral points. It will be noticed that simultaniously with the in- 

 crease in the diffusion of light by the dust in the upper atmosphere, 

 there was an increase in the antisolar distance of Arago's neutral 

 point with the sun above the horizon. With the sun below the 

 horizon, however, the intensity of the primary-diffused light from 

 the upper atmosphere, which was highly but not necessarily com- 

 pletely polarized, caused a decrease in the antisolar distance of 

 this neutral point. The minimum distance was not only smaller, 

 but it occurred with a lower sun than under normal conditions. 



With the decrease in the intensity of the primary-diffused sun- 

 light, in 1913, the antisolar distance of the neutral point gradu- 

 ally increased with the sun below the horizon, and the minimum 

 distance occurred with a higher sun. 



Jensen^ has shown that there may be a relation between varia- 

 tions in the color of skylight and the positions of the neutral 

 points. 



At the present time the solar and the antisolar distances of 

 the neutral points appear to be decreasing with the sun both 

 above and below the horizon. 



* Skylight polarization measurements made after sunset will be found tabulated 

 in Bull. Mount Weather Observatory, 6, 39, 1913. 



5 Jensen, Dr. Chr., tjber die grosse atmospherisch-optische Storung von 1912, 

 pp. 10-11 (Separatabdruck aus den Mitteilungen der Vereinigung von Freunden 

 der Astronomie und kosmischen Phj^sik). 



