154 HOWARD E. PULLING 



constant 5 of radiation is at present taken to be 1.93 calories 

 per square centimeter per minute, but the variations even from 

 day to day are distinctly noticeable. For example 4 the solar 

 constant for August 19, 1913, is given as 1.86 calories, whereas 

 the next day it appeared to be 1.99 calories. 



Before passing to a discussion of the distribution of energy 

 in the sun's spectrum it seems necessary to consider the dis- 

 tinction between prismatic and normal spectra, for the discussion 

 and graphs to follow are based upon the latter. 6 



Imagine a beam of light emitted by radiators of such numbers 

 and kinds that only waves between, let us say, 0.4 jU and 0.8/j. 

 are produced and that the intensity of radiation is the same for 

 all wave lengths, thus rendering the character of the radiation 

 as simple as possible. Imagine this beam to be passed through 

 an instrument that deflects the radiation of each wave length 

 along a line at right angles to their common path, the degree 

 of deflection being proportional to the wave length, and that 

 the instrument does nothing else whatsoever to the radiation. 

 Suppose further that this beam is projected upon a screen to 

 form a colored spectrum, which will be visible as a band. The 

 total energy incident upon the screen in unit time will thus be 

 exactly equal to the energy that would have been transmitted 

 to the screen in the same time if the instrument had done nothing 

 whatever to the beam, i.e., the beam has lost no energy through 

 deflection of the waves. Since the deflection is proportional to 

 the wave length and the energy transmitted in the wave lengths 

 were originally all equal it follows that equal areas in the illumi- 

 nated portion of the screen receive equal amounts of energy in 

 unit time. In other words, if the positions of the wave lengths 

 were marked on the screen as abscissas, and corresponding 

 ordinates were erected whose lengths are proportional to the 

 intensities of radiation on unit screen areas at several points, a 

 line joining their tops would be straight and parallel to the base. 

 Such a spectrum, with dispersions proportional to the wave 



6 Abbot, C. G., Pyrheliometry and solar radiation. Science N. S. 47: 609- 

 610. 1918. 



6 Baly, E. C. C, Spectroscopy. New York, 1905. 



Kayser, H., Handbuch der Spectroscopie. Bd. 1-6. Leipzig, 1900. 



