2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 69 



the points will be found to lie nearly on straight hues. The tangent 

 which the straight line best representing the points for the wave- 

 length A makes with the axis of abscissae gives log a w \ and its inter- 

 cept on the axis of ordinates gives log a„A . 



The observations taken each year at Mount Wilson ( generally dur- 

 ing the months from June to November, inclusive) have been sub- 

 jected, year by year, to this refining process. They yield the results 

 given in tables i and 2. The process is described in more detail and 



Table i. — Yearly Mean Dry-air Transmission Coefficients, a a -^ 

 Obtained at Mount Wilson, altitude 1,730 m., barometer 62.3 cm. 



Table 2. — Yearly Mean Atmospheric Aqueous-J'apor Transmission 

 Coefficients, a ^ 



plots shown in the Astrophysical Journal, 38, p. 392, 191 3, and 40, 

 p. 435, 1914. Improved apparatus and methods have led to greater 

 accuracy in plots like those of figure 1 of the first of these communica- 

 tions. Because of the presence in the upper atmosphere during 191 2 

 of a great amount of volcanic dust and the considerable variation of 

 its amount from day to day, the observations of that year were not 

 adapted to the present investigation and are omitted. 



