1890.] on Colour-Vision and Colour-Blindness, 123 



of the colour- sense. There is a very ingenious instrument, invented 

 by Mr. Lovibond, and called by him the " tintometer," which allows 

 the colour of any object to be accurately matched by combinations of 

 coloured glass, and to be expressed in terms 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 pigmentation 

 of the yellow spot, or possibly also to differences in the colour of the 

 internal lenses of the several eyes, the lens, as is well known, being 

 usually somewhat yellow after middle age. The differences are 

 plainly manifest in comparing persons all of whom possess tri-chro- 

 matic vision, and are not sufficient in degree to be of any practical 

 importance. 



The effect of the pigmentation of the yellow spot in modifying 

 colour may be rendered visible by an ingenious experiment, for which 

 we are indebted to Sir George Stokes. If the beam of the electric 

 lamp be suffered to pass through a cell containing a solution of 

 chloride of chromium [sAot(;w], and then to fall upon a white screen, 

 the green colour of the solution will be more absorbed by the yellow 

 spot than by the surrounding portions of the retina, and the result 

 will be the appearance of a faint roseate cloud floating in the centre 

 of the field. It would seem, from descriptions, that the pigmentation 

 of the yellow spot is more pronounced, and hence that the cloud is 

 more consjjicuous, in some individuals than in others. 



Taking the 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 them. Being insensible to one of the fundamental 

 colours of the spectrum, they must lose, roughly speaking, one-third 

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

 made good to them by increased acuteness of perception to the colours 

 which they see. Whether they see white as we see it, or as we see 

 tlie mixtures 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 any 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 coloured wools, and look at them, first through a glass 

 of peacock-blue, by which the red rays are filtered out [shoivn], and 

 next through a purple glass, by which a large proportion of the green 

 will be filtered out [s7«oifw], w^e may presume that, under the first con- 

 dition, the wools will appear 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 colour, but of different luminosity. 



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

 be apt to conjecture that the existence of colour-blindness in any 



