694 COLOR-VISION AND COLOR-BLINDNESS. 



differences in the pigmentation of the yellow spot rather than upon 

 any defect in the nervous apparatus of the color-sense. There is a very 

 ingenious instrument, invented by Mr. Lovibond and called by him the 

 " tintometer," which allows the color of any object to be accurately 

 matched by combinations of colored glass, and to be expressed in terras 

 of the combination. In using this instrument we not only find slight 

 differences in the combinations required by different people, but also 

 in the combinations required by the two eyes of the same person. Here 

 again, I think the differences must be due either to differences in the 

 l)igmentation of the yellow spot, or possibly also to differences in the 

 color of the internal lenses of the several eyes, the lens, as it is well 

 known, being usually somewhat yellow after middle age. The differ- 

 ences are plainly manifest in comparing persons all of whom possess 

 trichromatic vision, and are not sufficient in degree to be of any prac- 

 tical importance. 



Taking tbe ordinary case of a red-blind or of a green-blind person, 

 it is interesting to speculate upon the appearance which the world 

 must present to him. Being insensible to one of the fundamental 

 colors of the spectrum, he must lose (roughly speaking) one-third of 

 the luminosity of nature ; unless, as is possible, the deficiency is made 

 good to him by increased acuteness of percejition to the colors which 

 he sees. Whether he sees white as we see it, or as we see the mix- 

 tures of red and violet, or of green and violet, which they make to 

 match with it, we can only conjecture, on account of the inadequacy of 

 language to convey an accurate idea of sensation. We have all heard 

 of the blind man who concluded, from the attempts made to describe 

 scarlet to him, that it was like the sound of a trumpet. If we take a 

 heap of colored wools, and look at them first through a glass of pea- 

 cock blue, by which the red rays are filtered out, and next through a 

 purple glass, by which a large proportion of the green will be filtered 

 out, we may presume that, under the first condition, the wools will ap- 

 pear much as they would do to the red blind ; and under the second, 

 much as they would do to the green blind. It will be observed that the 

 appearances differ in the two conditions, but that in both, red and 

 green are practically undistinguishable from each other, and appear as 

 the same color, but of different luminosity. 



Prior to reflection, and still more, prior to experience, we should be 

 apt to conjecture that the existence of color-blindness in any individual 

 could not remain concealed, either from himself or from those around 

 him ; but such a conjecture would be directly at variance with the 

 truth. Just as it was reserved for Mariotte, in the reign of Charles II, 

 to discover that there is, in the field of vision of every eye, a lacuna or 

 blind spot, corresponding with the entrance of the optic nerve, so it 

 was reserved for a still later generation to discover the existence of so 

 common a defect as color-blindness. The first recorded case was de- 

 scribed to Dr. Priestley by Mr. Huddart, in 1777, and was that of a man 



