166 HOWARD E. PULLING 



by the molecules of the air and not by larger particles suspended 

 in the atmosphere, and calculated that the effect of these mole- 

 cules upon the light varied inversely as the fourth power of the 

 wave length. In table 4 are given calculations which Ray- 

 leigh made for the ratios between the intensities in a direct 

 beam of sunlight and those of skylight near some of the Frauen- 

 hofer lines, together with some of the ratios obtained by Abbot 27 

 on Mount Whitney. 



The calculations upon which Rayleigh's figures are based 

 assume that there is no other effective constituent of the at- 

 mosphere besides the permanent gases. This is, however, 

 not true in any actual case for, besides dust there is always 

 a certain amount of water vapor present. It has been found 2 * 

 that water in the atmosphere besides exerting selective ab- 

 sorption at certain regions of the spectrum also scatters light. 

 This effect of atmospheric water vapor increases as the wave 

 length decreases so that if its effect is added to the intensity 

 of the direct beam of sunlight, the observed figures in table 

 4 will still more closely approximate those calculations. The de- 

 crease in the ratios at about the D line is at least partly ex- 

 plainable by the presence in that region of a number of fine 

 aqueous lines located between the limits of about 0.535/* and 

 about 0.650/*. A graph exhibiting the non-selective transmission 

 by different depths of precipitable water is given in figure 3 

 and was plotted from Fowle's data. The abscissas signify 

 wave lengths and the ordinates the percentages of the radiation 

 transmitted in a direct beam from zenith sun. The depression 

 near 0.60/z indicates the selective absorption in the fine lines 

 mentioned above. 



Dust in the air reduces the transmitted energy by the amount 

 it absorbs and the amount it reflects outward into space. The 

 particles of dust are, however, so large relative to the wave 

 length of fight that they do not scatter the radiation as molec- 



27 Ann. Astrophys. Obs. Smithsonian Inst. 2: 156. 1908. 



28 Fowle, F. E., The non-selective transm^sibility of radiation through dry 

 and moist air. Astrophys. J. 38: 392-406. 1913. 



Idem as cited in note 14. 



