E. S. Ferry — Persistence of Vision. 201 



upon luminosity, duration of impression curves were obtained 

 from dichroic eyes. It is well known that a color-blind person 

 not only lacks one of the fundamental color sensations but also 

 that he perceives other colors differently from the normal. 

 For instance, according to Holmgren,* in the spectrum as seen 

 by red-blind persons yellow begins at about line C and extends 

 an orange, yellow and yellowish green and ends near F. At 

 this neutral zone the blue begins and extends to the end of the 

 normal spectrum. If now duration of retinal impression does 

 depend in any way upon color, one would expect that the 

 curves obtained from color-blind persons would differ from 

 those of the normal in a way that could not be explained by 

 considerations of luminosity alone. An examination of about 

 two hundred members of a large class in physics, by Holm- 

 gren's worsted method furnished eight cases of color-blindness, 

 one being red-blind, the remainder being green-blind. The 

 proportion of color-blind in even this limited number was 

 about the same as found by Dr. Jeffriesf from the examin- 

 ation of 175,000 persons. 



These color-blind students were examined for the neutral 

 point by the method of A. KoenigJ which consists in deter- 

 mining the color that they will match with white or gray. A 

 prism having one face coated with magnesium white was so 

 mounted in the Helmholtz color-mixing spectroscope, that a 

 ray of light from the collimator passing through the prism 

 would be dispersed into a spectrum, while a ray from a second 

 source falling on the white surface would be reflected directly 

 into the telescope. If now the eye- piece of the telescope be 

 removed, one-half the field of view will be filled with color and 

 the other half with pure white or gray. With this arrange- 

 ment a color-blind person will very accurately set the instru- 

 ment to the exact point where the two halves of the field of 

 view appear of exactly the same color to him. 



After their neutral points had been found in this manner, 

 these gentlemen very kindly offered to spend the time neces- 

 sary to obtain curves for their retinal persistence. 



Mr. W. C. "W. is a marked case of inherited blindness to 

 red. His father, uncles and brothers are similarly affected. 

 The red end of his spectrum ends at about "688 X, and his neu- 

 tral point is at "510 X. His eyes are otherwise normal. 



Mr. H. S. has inherited green-blindness from his maternal 

 relatives. Daltonism is so very rare among women that his 

 case was studied with great interest. He is making a specialty 



*How do the color-blind see the different colors? Proc. Roy. Soc. Loud., 

 xxxi (1880). p. 302. 



f3 - 95 per cent among males. "Color Blindness: its dangers and detection." 

 (Boston: 1879.) 



\ Zur Kentniss dichromatischer Farbensysteme. Wied. Ann , xxii (1884), p. 567. 



