no. 3 



ATMOSPHERIC SCATTERING OF LIGHT — FOWLE 



would vary directly as the sixth power of the diameter of the 

 scattering unit and the first power of N, so that diminishing N by -J-, 

 for instance, may increase the T 2 factor by 4- fold thus doubling the 

 scattering. In table 6 formula (2) has been used to avoid introduc- 

 ing the unknown index of refraction for atmospheric water vapor. 



Table 6. — The Variation of the Transparency of Atmospheric Water 

 Vapor compared with Solar Phenomena 



Arranging the figures of the 3d, 5th and 6th columns in order of 

 the increasing intensity of solar radiation the apparent correlation 

 of the three quantities is easily seen. 



Table 7. — Solar Phenomena and Atmospheric Water-Vapor Transparency 



Solar radiation. . 



N <>\--\ 



Sun-spot number 



1.955 

 0.34 

 60. 



The relationship between the solar constant of radiation, the sun- 

 spot numbers and the values of N tp (D' — D) 2 T 2 /D 2 is better shown 

 in figure 2, especially in the curves a, b, and c. Additional years of 

 observations will be required to thoroughly establish the relationship. 



SUMMARY 



The atmospheric transmission coefficients obtained at Mount 

 Wilson in the years 1910 to 1916 for the region free from selective 

 absorptions between wave-lengths 0.35 and 0.50 n have been analyzed 

 and have yielded the following data and results. 



The transmission coefficients for dry air (a a \) vary with the 

 inverse fourth power of the wave-length. They are apparently 

 wholly due to molecular scattering since the number of molecules 

 in a cm. 3 of air at 76 cm. pressure and o° C. corresponding to them, 

 (2.72 ±0.01 ) x io 19 , is in excellent agreement with the best value, 

 (2.705 ±0.003) X iq19 (Millikan), determined by other methods. 



