SUNLIGHT AND ITS MEASUREMENT 



i ;»:, 



lengths, is called a normal spectrum and a graph of the particular 

 one just considered to show its energy distribution is exhibited 

 in figure 1. 



If, however, the beam were passed through a prism the dis- 

 persions would not be in the same proportion for all wave lengths 

 with the result that the radiation of the longer wave lengths 

 would be more concentrated toward one end of the spectrum 

 and that of the short wave portion would be spread over a 

 greater area than in a normal spectrum of the same beam. The 



Fig. 1. Graphs of the distribution of energy in an hypothetical beam of light 

 when obtained from prismatic and normal spectra that cover the same area on 

 the screen. Ordinates represent intensities in arbitrary units; abscissas, rep- 

 resenting wave lengths, are noted on the curves. 



graph of a spectrum of the beam of light described above is 

 shown as the prismatic spectrum line of figure 1 . The apparently 

 higher energy content of the long red rays relative to the shorter 

 blue ones is now seen to be solely due to the fact that the latter 

 cover approximately nine times as much surface on the hypothet- 

 ical screen as the former. If the energy content at each wave 



