206 



E. S. Ferry — Persistence of Vision. 



the increase is nearly uniform for all wave-lengths. This fact 

 would be naturally expected, for it is well known that age de- 

 creases retinal sensitiveness ; and as the sensitiveness decreases 

 the action of the retina would be less quick either to receive 

 an impression or to dismiss one. 



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Summary of Results. 



I. The duration of retinal impression is very different for 

 different regions in the spectrum, being at a minimum value at 

 the region of maximum luminosity and gradually increasing to 

 maximum values at the ends of the spectrum. 



II. If the luminosity of any region in the spectrum be so 

 changed that the values vary in geometrical ratio, the corre- 

 sponding values of duration of impression will approximately 

 vary in arithmetical ratio for regions of ordinary brightness. 



III. Color has, at most, very slight influence upon retinal 

 persistence. Luminosity, — including the brightness of the light 

 and the retinal sensitiveness — is the all-important factor. 



