[allbn] 



PHENOMENA OF THE PERSISTENCE OF VISION 



197 



three, five and ten minutes. Observations were made upon the same, or 

 nearly the same, portions of the spectrum a& the fatiguing colours. The 

 mieasurements are shovm in table 2 and are plotted in figure 2, accord- 

 ing to a convenienit scale of numbers so chosen that the curves are all 

 exhibited in one figure. The zero of each curve, which corresponds to 

 the normal persistence of the particular colour, is the intersection of the 

 curve with the axis of ordinates. 



With all colours the fatiguing effect reaches its maximum in about 

 three minutes, further exposure to light seeming fto make no difiEerence 

 with tilie duration of the light impressions. 



Fig. 2. 



Perhaps the m'ost remiarkable curves are those for the yellow and 

 the blue colors, whose wave lengths are .575 // and .472 p. respec- 

 tively. The Latter of these is a straight line and the former nearly so. 

 For these colours the retina does not seem capable of being fatigued, 

 as the duration of the colour impressions always retains its normal 

 value. The differiences between normal and maximum readings of the 

 viarious curves vary in magnitude, and are shown in figure 3. This 

 curve is characterized by three elevations, corresponding to the red, green, 

 and violet of the spectrum, which are separated by two depressions cor- 

 responding" to yellow and blue of wave lengths .575 p. and .472 /i 

 respectively. , 



