3o6 LIGHT AND THE BEHAVIOR OF ORGANISMS 



obtained results which agree fairly well with those of 

 Fraunhofer, by means of finding the amount of white light 

 required to make a given color imperceptible when added 

 to it. The results of Vierordt are graphically given in 

 Fig. 34. 



400 460 520 580 G40 700 760 



Fig. 34. Curves representing the relative distribution of brightness and energy 

 in the spectrum, i, brightness curve for color-bHnd persons showing that the 

 red end is considerably shortened (after Konig, i8gi); 2, brightness curve for 

 normal eye in prismatic solar spectrum (from Davenport, 1897, p. 159, after Vier- 

 ordt, 1873, p. 17); 3, 4, and 5, brightness curves for normal eye in normal gas- 

 light spectrum of low, medium, and high intensities respectively (after Haycraft, 

 1897, p. 141); 6, energy curve for solar prismatic spectrum (after Langley, 1884, 

 P- 233); 7, energy curve for normal gas-light spectrum constructed from data 

 given by Nichols, 1905, p. 151. The value of the ordinates in most of these curves 

 is arbitrary. A-G, approximate positions of Fraunhofer lines in spectrum. 



These curves show clearly that the distribution of brightness in the spectrum is 

 not proportional to the energy of the diflferent parts and that consequently bright- 

 ness must depend upon the length of the waves as well as upon their amplitude. 



Haycraft (1897, p. 140) ascertained the brightness dis- 

 tribution in a normal gas spectrum produced with Hilgar's 

 large spectroscope and diffraction grating, by means of 

 the so-called flicker method. The results obtained by this 

 method, graphically recorded in Fig. 34, agree very well 



