158 COLOR-BLINDNESS IN ITS RELATION TO 



should be applied the name of color-blindness, and which, owing to their 

 nature theoretically, must be considered as of different kinds. This 

 division will be sanctioned if we consider the relations in which it stands 

 to the method pursued for discovering them, and which is based on the 

 Young-Helmholtz theory. It is this we are about to explain. 



We classify the different kinds of color-blindness under especial heads, 

 to be able the better to grasp the whole. We might indeed divide this 

 blindness into congenital and acquired, but as such a division has refer- 

 ence alone to the mode of origin, and not to the nature of this blindness, 

 and affects in no wise the manner of its discovery, it has no practical 

 importance in the case now occupying our attention. Besides, our 

 division relates, as does our entire memoir on this subject, essentially to 

 congenital color-blindness. The division is as follows : 



I. Total color-blindness {total farghlindhet), in which the faculty of 

 perceiving colors is absolutely wanting, and where the visual sense con- 

 sequently can only perceive the difference between darkness and light, 

 as well as the different degrees of intensity of light. 



II. Partial color-blindness {partiel farghlindhet), in which the faculty 

 of certain perceptions of color, but not of all, is wanting. It is subdivided 

 into — 



1. Complete or typical color-blindness (fullstandig or typisJc farg- 

 hlindhet), in which one of the three fundamental sensations, one of the 

 three perceptive organs of color in the retina, is wanting, and in which 

 consequently the colored visual field has but two ranges. This group 

 includes three kinds, namely : 



[a) Red-blindness {red hlindhet). 

 {h) Green-blindness {gron hlindhet). 

 (c) Violet blindness {violett hlindhet). 



2. Incomplete color-blindness {ofullstdndig farghlindhet), where one 

 of the three kinds of elements, or perhaps all, are inferior in excitability 

 or in numbers to those of the normal chromatic sense. Incomplete color- 

 blindness exhibits, like the normal sense, three zones in the visual field, 

 but is distinguished from it by an unusually small central field. This 

 group includes the whole of a series of different forms and degrees, a 

 part of which — the superior degrees, which might be called incomplete red- 

 hlindness and incomplete green-hlindness (and incomplete violet-hlindness) — 

 constitutes the transitions to the corresponding kinds of complete color- 

 blindness, and another part of which — the inferior degrees, which we call 

 a feeble chromatic sense — constitutes the transition to the normal sense 

 of colors. 



We will show further on that this classification, based entirely upon 

 the Young-Helmholtz theory, is quite practical, and conformable to 

 experience. We know no classification which, though distinguishing 

 accurately between the different essential forms of a defective sense of 

 colors, draws a surer, more decided, and more practical limit between 

 the defective sense of colors and the normal sense. 



