192 Dr. W. Pole on Colour-Blindness. 



The author points out that if we suppose the red and green 

 elements to be removed from normal vision, the blue, yellow, 

 white, and black sensations remaining as before, then the 

 impressions given by the spectrum will be exactly as they 

 are described in the colour-blind cases. The patient will see 

 the first half of the spectrum a pure yellow, and the second 

 half a pure blue ; but both will be modified in different 

 places with different nuances and degrees of saturation. 



Before reaching the point which corresponds to the normal 

 fundamental green the yellow will gradually fade in the 

 presence of the colourless or white sensation, and beyond this 

 point the blue will enter and gradually increase in a corre- 

 sponding manner. 



Such a colour-blind person will not be actually blind to the 

 pure green of the normal eye; he will see it as a colourless 

 sensation. And on the other hand the rays of the spectral 

 red will be visible to him so far as their yellow and white 

 " Yalenzen " are strong enough to make an impression. 

 His spectrum will therefore be divided into a first yellow 

 and a second blue half ; and as all the various rays of each 

 half will only be distinguished by various additions of black 

 or white, and not by difference of hue, all his colour-sensations 

 may be imitated by suitable mixtures of yellow or blue with 

 white, black, or grey. Also as his two hues must be comple- 

 mentary, mixtures of the two together in the proper pro- 

 portions will be colourless. 



This description assumes that the yellow, blue, and white 

 effects are the same for the colour-blind as for the normal 

 eye. It is, however, found that these may vary in different 

 colour-blind individuals [as indeed they do in the normal 

 eye], and this has given occasion to divide such patients into 

 two classes which the adherents of the Young-Helmholtz 

 theory separate as red-blind and green-blind. But the author 

 has tried patients of both classes, and has found his explana- 

 tion equally applicable without exception. 



The author then describes experiments he has made with 

 the spectrum. The colourless spot lies, as is well known, 

 between b and F ; it is not always in the same place, but 

 lies sometimes nearer b and sometimes nearer F. It is pos- 

 sible that an analogous difference of place may exist in normal 

 eyes ; for in the place where one person may see a pure green 

 another may see a blue-green or a yellow-green, and so on. 



The position of the whole blue and yellow spectrum de- 

 pends on the position of the neutral spot. The farther this 

 spot lies towards the blue so much farther is the beginning 

 of the yellow spectrum shifted in the same direction, so 



