E. /S. Ferry — Persistence of Vision. 205 



produced. Fechner's law was also empirically deduced and 

 has since been confirmed, for mean values, by Dalbouf's 

 memoir to the Belgian Academy. It can be expressed in the 

 form 



s = h l • log x 



where s denotes intensity of sensation and x intensity of stim- 

 ulus. In this particular case the stimulus is luminosity, hence 

 we have 



K l( ' S 



which means simply that retinal persistence varies inversely as 

 the intensity of the sensation producing it. This seems to agree 

 with ordinary experience and thus to confirm the validity of 

 the law connecting retinal persistence and luminosity. 



Effect of Age upon Retinal Persistence. 



It was thought that possibly as a person advanced in years, 

 the retina might become selective in its sensitiveness for differ- 

 ent colors and that therefore the curve of duration of retinal 

 impression might be different from that of a younger person. 

 Two professors in the University kindly permitted their eyes 

 to be tested for retinal persistence and the values obtained are 

 given in the annexed table. 





Table VII. 





Wave-length. 



Duration of retinal 



impression in second 



A 



Prof. S. G. W. 



Prof. E. L. N. 



•435 



•041V 



•0417 



•480 



•033;* 



•0357 



•510 



•0217 



•0227 



•540 



•0179 



•0185 



•570 



•0156 



■0161 



•589 



•0147 



•0155 



•615 



•0156 



•0166 



•645 



•0185 



•0 1 92 



•684 



•0227 



•0227 



These values platted in fig. 5 seem to indicate that for both 

 Dr. Nichols and Prof. Williams the more refrangible part of 

 the spectrum is proportionately less luminous than to the eyes 

 assumed to be normal. But the violet end of the spectrum is 

 so feeble that observations in it are very difficult, and cer- 

 tainty cannot be obtained without more extended observa- 

 tions. If anything can be deduced from so few observations, 

 these curves show to a high degree of probability that age in- 

 creases retinal persistence to a considerable amount and that 



