198 E. 8. Ferry— Persistence of Vision. 



Table II. 



Duration of retinal impressions for monochromatic light of 

 different wave-length, of relative brightness from L to 24. 



Wave- Duration of retinal impressions in seconds, 



length. 1. 2. 4. 8. 16. 24. 



■435 -0357 -0294 '0227 



•480 -0328 0286 -0250 -0217 



•510 -0200 



•540 -0200 -0192 -0172 "0156 "0133 01 19 



•570 0139 -0109 



•589 -0170 -0161 -0147 -0132 '0102 "0081 



•615 0141 -0111 



•645 "0204 -0192 -0179 -0156 0130 



•684 -0238 -0217 "0192 "0172 "0156 



These values are platted in the curves shown in fig. 1. The 

 numbers affixed to the curves indicate the relative brightness 

 of the spectra. 



These curves show that with increased brightness the values of 

 retinal presistence do not shift their positions relative to wave- 

 length ; that as the brightness of the spectrum increases, the dur- 

 ation becomes less in such a manner that each point in the curve 

 is shifted downward by a nearly constant amount ; and that the 

 distance separating the different curves has a definite relation 

 to the difference of the light intensity of the spectra from 

 which the curves were obtained. 



If the values be noticed for the duration of impression of all 

 the curves, corresponding to any single wave-length, it will be 

 perceived that the following statement is approximately true : 

 as the intensity of light increases in geometrical ratio, the 

 duration of the corresponding retinal impression decreases in 

 arithmetical ratio. This statement can be concisely expressed 

 in the form of the approximate empirical law — the difference 

 of the duration of two retinal impressions produced by two 

 lights of the same color, is inversely proportional to the 

 logarithm of the quotient of the respective luminous inten- 

 sities. 



The value obtained for the ends of the spectrum deviate 

 from this law, but this is probably due to the uncertainty of 

 the observations in these faintly illuminated regions. The 

 relation between duration and light intensity, thus far deals 

 simply with lights of the same color. The next object of the 

 experiment was to test the generality of this law, by determin- 

 ing if it would hold for lights of different color. 



Relation between Luminosity and Duration of Retinal Im- 

 pression. 



By luminosity is meant the physiological effect of light upon 

 the eye by means of which vision is accomplished. The meas- 



