202 HOWARD E. PULLING 



sitivity curve is expressed as unity and all its other ordinates are 

 likewise fractions. 



If the two maxima coincided in postion on the scale of ab- 

 scissas, unit darkening would be produced in unit time by the 

 action of this maximally effective wave length when acting by 

 itself. If the maximum sensitivity corresponds in position 

 with a wave length of half the efficiency of the maximum, the 

 degree of darkening in unit time will be one-half the maximum 

 (unity) noted above; or if the maximum efficiency coincided 

 in position with half the sensitivity the darkening would again 

 be one half the maximum. In each case it is supposed that only 

 those waves are acting whose ordinate values are considered. 



The graph of the darkening produced in unit time by the 

 radiation characterized by line B acting upon the paper having 

 a curve of sensitivity, A, will accordingly be obtained by plotting 

 the products of the ordinates of radiation and sensitivity at 

 each wave length. The result is expressed in line C (plotted 

 on a larger scale of abscissas). The total amount of darkening 

 will be proportional to the area between line C and the base line, 

 and if the darkening is to be a measure of solar energy, the areas 

 enclosed by the curve of darkening and by the curve of radiation 

 intensity must always bear a fixed relation to each other, since 

 the former represents amount of darkening and the latter the 

 amount of radiation incident upon the same surface in the same 

 length of time. Of course only the radiation to which the 

 paper is sensitive can be effective so, unless the remaining 

 radiation bears a fixed relation to the effective portion, darkening 

 cannot be a measure of the total radiation incident in unit time. 



To find if this relation exists for the sun's radiation it is only 

 necessary to compare the areas below different solar radiation 

 curves in the different portions considered. To make it more con- 

 crete still, a darkening curve for solar radiation with the sun 

 15° above the horizon at sea level (curve D) is plotted as line E. 

 The area between fine E and its base (curve of darkening for 

 15° sun) is 0.19 of that between line C and its base (curve of 

 darkening for 90° sun), so that the total radiation in the two 

 cases should be in the same ratio if the actinometric method 

 is to be useful for their measurement. 



