BELL. — THE PHYSIOLOGICAL BASIS OF ILLUMINATION. 



91 



illumination is progressively diminished, color vision gets more and more 

 imperfect and uncertain, especially toward the red end of the spectrum. 

 The effect is shown very clearly in the variation of Fechner's fraction 



with color as the intensity changes. Figure 5 shows the change in -=■ 



with X for intensities of 15 meter-candles (a) and 0.75 meter-candles 

 (b) respectively from the data obtained by Konig and Brodhun (loc. 



,60 

 .-50 

 .40 

 .30 

 .20 

 .10 



700/U./X. 



GOO 



A 

 Figure 5 



500 



400 



cit.). Looking at the latter, it is evident that for the orange and red, 

 vision must be very poor indeed, and in no part of the spectrum really 

 good. In curve a color vision is pretty well established, although there 

 are still traces of the point of inflection, which, as we shall presently see, 

 falls near the point of maximum sensitiveness in very weak light, as if 

 the superimposed rod vision were still helping out at this moderate 

 intensity. 



The Purkinje phenomenon, now well known to depend on the pro- 

 gressive failure of cone vision, also gives valuable evidence along the 

 same line. It was noticed more than twenty years ago by Professor 



