E. S. Ferry — Persistence of Vision. 



203 



curves in fig. 4. The normal curves are drawn in as before to 

 show the relation between the normal and color-blind eyes for 

 persistence of vision. 



Table VI. 



Duration of Retinal Impression for Dichroic Eyes. 



{The numbers at the head of each column indicate the brightness of the spectrum.) 





Duration of Impression in Seconds. 





Red Blind. 



Green Blind. 





Wave- 

 length. 



Mr. W. C. W. 



Mr. H. S. Mr. G. A.W. Mr. W. M. Mr. H. C. ». 



Mr.L. M.W. 



A. 



4. 



8. 



4. 



2. 



4. 



"8. 



•435 







-0357 





•0357 



■480 



•0294 



•0357 



•0278 -0333 



•0294 



•0250 



•510 



0227 



•0227 



•0227 -0263 "0217 



•0192 



•525 





•0208 -0200 -0238 "0208 



•0185 



•540 



•0172 



•0200 -0185 -0222 "0200 



•0172 



•555 



0161 



•0175 -0179 -0208 -0193 



•0156 



•570 



•0156 



•0161 



•0161 -0192 -0166 



•0143 



•589 



•0152 



•0143 



•0152 0159 -0151 



•0125 



■615 



0179 



•0154 



•0156 -0175 -0161 



•0143 



•645 



•0217 



■0175 -0167 -0192 -0172 



•0156 



•684 



• 



0210 -0227 -0238 -0217 



•0179 



These curves show that light impressions of red last much 

 longer on the retina of red-blind persons than on the normal, 

 yellow somewhat longer than normal and the other colors about 

 the same as normal. With green-blind persons, however, green 

 impressions persist much longer than normal, red a little less 

 than normal and the other colors the same as normal. 



An explanation of the difference between the duration 

 curves of dichroic and normal eyes was found in the difference 

 in the sensitiveness for different colors of the dichroic from the 

 normal eye. Messrs. Mace and Nicati* from the examina- 

 tion of a number of dichroic eyes obtained luminosity values 

 which indicate, first, that red-blind people perceive red weakly, 

 yellow nearly normal, green better than normal ; second, that 

 the green blind have better perception than normal for red, 

 green feeble, yellow and blue normal. A later determination 

 by Abney and Festingf confirmed their conclusions. In this 

 experiment luminosity values were also obtained from Mr. W. 

 C. W. which agree with the two determinations just cited. 

 This shows that if account be taken of the difference between 

 the sensitiveness of the normal and the dichroic eye for different 



* De la distribution de la lumiere dans le spectre solaire (spectre des Daltoniens). 

 Comptes Rendus, xci, p. 1078. 



f Colour Photometry. Trans. Roy. Soc. Lond., 1888, p. 547. 



