692 COLOR-VISION AND COLOR-BLINDNESS, 



tbis latter region, such coues as may exist are uot sensitive to any color, 

 but, like the rods, only to difterences 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 color 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 color-sense which in the human race (or at least in 

 civilized man) exist normally in outer zones of the retina, are found in 

 a few individuals, to exist also in the center. There are persons in whom 

 the region of the yellow spot is absolutely insensitive to color, and 

 recognizes only diflerences in the amount or quantity of light. To such 

 persons the term " color-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 color-blind would see a colored picture as if it were 

 an engraving, or a drawing in black and white, and would perceive dif- 

 ferences between its jjarts only in the degree in which they differed in 

 brightness. 



A more common condition is the existence, in the center of the retina, 

 of a kind of vision like that which normally exists in the zone next sur- 

 rounding it ; that is, a blindness to green. Persons who are blind to 

 green appear to see violet and yellow much as these are seen by the 

 normal-sighted, and they can see red, but they can not distinguish 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 colors may be either couj- 

 plete or incomplete ; that is to say, the power of the color 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 color-sense." 



The experiments of Helmholtz upon color led him to supplement the 

 original hypothesis of Young by the supposition that the special nerve 

 elements excited by any one color are also excited in some degree by 

 each of the other two, but that they respond by the sensation appropri- 

 ate to themselves, and not by that appropriate to the color 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 fibers 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 fibers, and stimu- 

 lates slightly both the red-perceptive and the violet-i)erceptive — result- 

 ing sensation, green. Pure violet stimulates strongly the violet-percep- 

 tive fibers, less strongly the green-perceptive, least strongly the red- 



