156 COLOR-BLINDNESS IN' ITS RELA.TION TO 



point. Outside of the central field extends a belt which surrounds it 

 on all sides, and in which our whole system of colors is classed under two 

 heads, exactly as in the case of the red-blind. We have here, as in the 

 last case, but twocolors, yellow and bluQ. In other words, we are completely 

 red-blind in this intermediary zone; beyond this, there is a peripheral belt, 

 in which we are totally color-blind. These are matters unquestionably 

 of great theoretic value, but it must be acknowledged they are also of 

 great practical Importance. Although we have adopted the Young-Helm- 

 holtz theory, we must admit that the different kinds of perceptive elements 

 of colors have a different local division upon the retina ; and this is why 

 we may speak of the topography of the chromatic sense. The fact is ex- 

 plained in this way : in the retina ot the normal eye, there are simultane- 

 ously three kinds of elements in the central part, corresponding to the 

 central region of the visual field. Toward the periphery, beginning at 

 the central fossa, the elements become more and more rare, but in un- 

 equal proportions, so that the perceptive organs of red cease first, and 

 this at a limit corresponding to that of the central region. In a belt 

 which answers to the intermediate zone, or the belt of the red-blind, 

 there remains in consequence but the perceptive elements of green and 

 violet. At the limit near the periphery of the retina, corresponding to 

 that of the peripheral zone of the visual field, or region of absolute 

 color-blindness, the perceptive elements of green cease also, so that there 

 only remains in this last zone the perceptive elements of violet. We 

 have been especially led to this last conclusion by the examination of 

 two cases of color-blindness, where the visual field was so abnormally 

 small that the peripheral zone seemed to be effaced, and where we be- 

 sides recognized the characteristic features of violet-blindness. This 

 experiment, which perfectly harmonized with the theory, showed us the 

 relation of complete red-blindness to the normal chromatic sense. Eed- 

 blinduess is distinguished from the normal sight in this, that the normal 

 central field is wanting, but is replaced by a mean corresponding at the 

 same time to the central field and to the intermediary zone of the normal 

 sight. We have also succeeded, owing to the peripheral investigation 

 of the colored visual field, in verifying in a great number of cases the 

 continuous series of forms of transition which we have classified as one 

 kind under the head of incomplete color-blindness, or in the instance es- 

 pecially occupying us here, incomplete rcd-hlindness. In the same way, 

 the other kinds of color-blindness may also, as regards the visual field, 

 be classified according to the theory. The visual field of the green-blind 

 is distinguished from that of the normal observer in this, that it has a 

 peripheral field corresponding in extension both to the intermediary and 

 peripheral zones of the normal observer. The violet-blind is distinguished 

 on the other hand in this, that it is completely deficient in the normal 

 peripheral zone. These two kinds of incomplete color-hlindness are charac- 

 terized by a central field diminished at every degree. With regard to 

 the visual field we may therefore lay down this rule, that it has as many 

 distinct zones, with reference to the perception of colors, as the chro- 



