E. 8. Ferry — Persistence of Vision. 193 



Cavallo,* and Parrottf have since fully substantiated this value 

 by accurate measurements. Plateau,:}; in 1829, found that when 

 discs divided into sectors alternately black and of some special 

 color were rotated, different speeds were required to produce 

 uniformity of tint, depending upon the color • of the pigment 

 used to paint the alternate sectors. By noting the angular 

 size of the black and the colored sectors and the speed just 

 necessary to produce uniformity of tint, he was enabled to de- 

 termine the absolute duration of the maximum impression for 

 the different colors experimented upon. By using the differ- 

 ent discs painted in sectors, each having black alternating with 

 some one color, Plateau obtained the following values for the 

 duration of impression for these particular colors : — 



White 0-191 sees. 



Yellow 0-199 " 



Red 0-232 " 



Blue 0-295 " 



One great difficulty with this method is that since no pig- 

 ment gives a pure color and since pigments vary so widely in 

 tint, the results obtained hold only for the particular speci- 

 mens experimented upon. To overcome this difficulty, Dr. E. 

 L. Nichols§ employed a revolving disc having sectors cut out, 

 in front of the slit of the spectroscope, and defined his colors 

 by their wave-lengths. In this manner by suitably choosing 

 the wave-lengths used for observation, a curve was drawn 

 showing the relation between the duration of impression and 

 the color corresponding to any wave-length. 



From the results obtained by Plateau, Emsmann,|| Nichols 

 and others, it was reasoned that the duration of retinal impres- 

 sions depends upon the intensity of the light-giving source 

 and upon the color of the light entering the eye. To test the 

 validity of this latter proposition, and to determine the princi- 

 pal factors producing persistence of vision has been the object 

 of the series of experiments now to be described. 



Apparatus and Method of Observation. 



The plan of the investigation was to obtain curves showing 

 the relation between duration of the retinal impression of the 

 normal eye and wave-length of light observed for spectra of 



* The elements of natural or experimental philosophy (London, 1803), vol. iii, 

 p. 135. 



f Entretiens sur la physique (Dorpat, 1819), vol. iii, p. 235. 



\ Dissertation sur quelques proprietes des impressions produites par la lumiere 

 sur l'organs de la vue. (Liege, 1829). 



§ On the Duration of Color Impressions upon the Retina, this Journal, vol. 

 xxviii, p. 243. 



I Ueber die Dauer des Lichteindrucks, Pogg. Ann., xci (1854), p. 611. 



Am. Jour. Scl— Third Series, Vol. XLIV, No. 261.— Sept., 1892. 

 13 



