BELL. — TYPES OF ABNORMAL COLOR VISION. 



involved with the residuum of the red and green sensations, the yellow- 

 ing of the lens, and variations of macular pigmentation, are matters 

 to be considered when the simpler facts of sensation can be better co- 

 ordinated. 



The relations of the three fundamental color sensations of the Young- 

 Helmholtz theory, at least rest on a sound experimental basis and it is 

 well understood that ordinary cases of color blindness imply an easily 

 measurable deficit in one of these sensations. 



In the normal or average eye the three fundamental sensations are 

 related in a certain normal manner which may be schematically repre- 

 sented as 



R, G, B, 



in which each of the sensations has its typical average value. Simi- 

 larly the ordinary case of red blindness may be written — R, G, B 

 indicating a deficit from the normal value of the red sensation. To be 

 rigorous one should give R a coefficient indicating the relative deficit 

 of the red vision from its normal which may be anything from an 

 amount just recognizable as a variant from normal to 100%. For 

 example a certain case of partial red blindness examined by the writer 

 would have been expressed 



-.64R, G, B. 



And similarly one may set down as the variants from normal involving 

 change in a single color sensation curve the following 



R, G, B, 

 (1)+R, G, B, 



(2) R.+G, B, 



(3) R, G,+B, 



(4) -R, G, B, 



(5) R,-G, B, 



(6) R/ G,-B, 



Of these (4), the — R type, is the ordinary case of "color blindness"; 

 (5) is the rather rare green blindness of which typical cases are reported 

 by Abney (Proc. Roy. Soc, Vol. 83 A); (6) is the still rarer blue blind- 

 ness (Abney, Colour Vision, p. 73). Of the plus variants (1) is the 

 plus red, the first of the type detected, (Rayleigh, loc. cit.). Burch 

 (Physiological Optics, p. 119) cites several instances of (2). The plus 

 blue variation is apparently very uncommon but Burch cites a case 



