1890.] on Colour- Vision and Colour-BUndness. 121 



the rods, only to differences in the amount of light. When cones 

 of only one kind are called into activity, the sensation produced 

 is named red, green, or violet ; and, when all three varieties are stimu- 

 lated in about an equal degree, the sensation produced is called white. 

 In the same way, the innumerable intermediate colour-sensations of 

 which the normal eye is susceptible must be ascribed to stimulation 

 of the three varieties of cones in unequal degrees. 



The conditions of colour-sense, which, in the human race, or at 

 least in civilised man, exist normally in outer zones of the retina, 

 are found in a few individuals to exist also in the centre. There 

 are persons in whom the region of the yellow spot is absolutely 

 insensitive to colour, and recognises only differences in the amount 

 or quantity of light. To such persons, the term "colour-blind" 

 ought perhaps in strictness to be limited ; but the individuals in 

 question are so rare that they are hardly entitled to a monopoly of 

 an appellation which is conveniently applied also to others. The 

 totally colour-blind would see a coloured picture as if it were an 

 engraving, or a drawing in black and white, and would perceive 

 differences between its parts only in the degree in which they 

 differed in brightness. 



A more common condition is the existence, in the centre of the 

 retina, of a kind of vision like that which normally exists in the zone 

 next surrounding it — that is, a blindness to green. Persons who are 

 blind to green appear t(» see violet and yellow much as these are seen 

 by the normal-sighted ; and they can see red, but they cannot dis- 

 tinguish it from green. Others, and this form is more common than 

 the preceding, are blind to red ; and a very small number of persons 

 are blind to violet. Such blindness to one of the fundamental colours 

 may be either complete or incomplete — that is to say, the power of 

 the colour in question to excite its proper sensation may be either 

 absent or feeble. In some cases, the defect is so moderate in degree 

 as to be adequately described by the phrase " defective colour-sense." 



The experiments of Helmholtz upon colour led him to supplement 

 the original hypothesis of Young by the supposition that the special 

 nerve elements excited by any one colour are also excited in some 

 degree by each of the other two, but that they respond by the sensa- 

 tion appropriate to themselves, and. not by that ap23ropriate to the 

 colour by which they are thus feebly excited. This, which is often 

 called the Young-Helmholtz hypothesis, assumes that the pure red 

 of the spectrum, while it mainly stimulates the fibres sensitive to 

 red, stimulates in a less degree those which are sensitive to green, 

 and in a still less degree those which are sensitive to violet, 

 the resulting sensation being red. Pure green stimulates strongly 

 the green-perceptive fibres, and stimulates slightly both the red-per- 

 ceptive and the violet-perceptive — resulting sensation, green. Pure 

 violet stimulates strongly the violet-perceptive fibres, less strongly 

 the green-perceptive, least strongly the red-perceptive — resulting 

 sensation, violet. When all three sets of fibres are stimulated at 



