KNOWLEDGE AND OPINION IN REGARD TO COLOUR-BLINDNESS. 445 



colour enters, at first much diluted with white, but gradually strengthening till it becomes brilliant 

 and fully saturated between F and G. Beyond this it is gradually darkened, but still preserving its 

 saturation till it disappears. 



In order to afford some illustration of these appearances, I have attempted to give, in 

 the Plate, fig. 2, a chromo-lithographic imitation of the drawing which I prepared, with 

 Dr Huggins's aid, in 1879; adding in fig. 1 a corresponding indication of the places 

 of the normal spectral colours. But it must be understood that such sketches are 

 necessarily" very imperfect representations either of the colours, or the luminosity. 



The extra spectral hues, which were conceived by Newton to connect the red end of 

 the normal spectrum with the violet end, and are now called " purples," are also visible 

 to the dichromic eye ; those near the red have the warm colour ; those near the violet 

 have the cold colour ; — the place of separation being a second Neutral or colourless Point 

 near the red end, which is to the normal eye a deep crimson or red violet. 



I add a diagram, fig. 3, containing three coloured squares. These all appear alike (or 

 very nearly so) to me, and I should describe the hue of each as a colourless grey of 

 moderate intensity. The middle one corresponds, I am told, fairly well with the normal 

 neutral ; the right-hand one represents my neutral hue in the green ; and the left-hand 

 one represents my second neutral hue among the extra-spectral purples.* 



According to many trials with different persons, I see the full length of the spectrum 

 ordinarily visible at both ends, the red end being perfectly distinct, as far as it is visible 

 to any normal-eyed person who has tried it with me. The violet end seems often to vary 

 with normal eyes. 



As to the luminosity in different parts of the spectrum, Dr Huggins made some trials 

 with me in 1879, and we found that my impressions of the gradations, throughout, seemed 

 to agree fairly well with the normal ones, as originally determined by Fraunhofer in 

 1815; the maximum being, as he expressed it, at about " ^ or £ DE, from D towards 

 E," a place which he expressly marked as yellow. How far the actual intensity of the 

 luminous impression on my eyes may compare with the normal one (which may probably 

 itself vary), I know no means of determining ; all I can say is that my two colours appear 

 to me exceedingly vivid and brilliant, and that 1 find by experience I am more sensitive 

 than many normal-eyed people to slight variations in them. 



It must be further explained that although the dichromic patient, as his name implies, 

 sees only two varieties of hue, yet the colour-impressions he derives from them are very 

 numerous and diversified in character, and this is the reason why his defect so often escapes 

 notice. He hears of the variety in colour presented by nature, and he knows that nature 

 also offers great variety to him ; but he does not know, till he is taught, that the variety 

 is of a different kind in the two cases. In normal vision there is an immense variety 

 of hues ; in dichromic vision, though the colours are fewer, they are still subject to 



* The exact matching of these three colour-impressions will vary slightly in different dichromic eyes, for reasons 

 hereafter given ; with some patients the external squares in the figure will appear slightly coloured, and varying in 

 intensity. Indeed the appearances may vary slightly, even in the same individual, with variations of the illumination. 



