SUNLIGHT AND ITS MEASUREMENT 



201 



or the percentages of the total darkening produced by this small 

 portion of the spectrum will be equal to the quotient obtained 

 by dividing the small area by the entire area first mentioned. 

 If, as is usally the case, the incident radiation is not of equal 

 intensity at the various wave lengths, the graph of radiant 



Fig. 5. Graphs exhibiting the effects of solar radiation upon silver-chloride- 

 gelatine preparations. Line A : graph of sensitivity of a silver-chloride-gelatine 

 preparation. Line B: graph of solar radiation; sun at 90° altitude. Line C: 

 graph of effect of radiation represented by line B upon the photo-sensitive prep- 

 aration. Line D: graph of solar radiation; sun at 15° altitude. Line E: graph 

 of effect of radiation represented by line D upon the photo-sensitive preparation. 



energy will also be a curve, and the resultant darkening will 

 depend upon the overlapping portions of both curves. Line B 

 in figure 5 represents an energy curve of solar radiation re- 

 ceived at sea level with zenith sun. The maximum ordinate 

 is expressed as unity so that all other ordinates are fractions of 

 this maximum. Similarly the maximum ordinate of the sen- 



