THEORIES OF COLOR VISION 749 



from 4 to 5 per cent, of men and about i per cent, of women are defective in 

 color vision. The defect is called color-blindness. 



In very rare cases complete color-blindness exists. Such individuals 

 distinguish lights and shades only, that is, form. A more common defect, 

 however, is the absence of one or more of the fundamental color sensations, 

 the most common of all being the green blind, or the red-green blind. The 

 red-green blind individual cannot distinguish red and green colored yarns 

 from each other or from shades of gray which reflect light with the same 

 intensity. When they are given the color test by the Holmgren yarns, they 

 indiscriminately mix the reds, greens, and grays. Cases have been described 

 in which the individual was red-blind alone, or green-blind alone. A less 

 common color defect is the inability to distinguish yellows and blues, yellow- 

 blue blindness. 



Color-blindness may occasionally arise from disease or accident, but it 

 is usually congenital. The individual often does not discover his defect until 

 examined especially for his color vision. He may have learned to apply 

 the terms green and red to surrounding objects, such as the grass, bricks, etc., 

 but he distinguishes these objects by slight differences in intensity of lumi- 

 nation, form, etc., and not by the sensations of color which the normal 

 individual experiences. 



Theories of Color Vision. We have no way of determining the 

 method by which the colors stimulate the retina other than our inferences 

 from indirect evidence. It is probable that the energy of light vibration 

 is transformed in the retinal structures into either physical or chemical 

 change, perhaps the latter. Those interested in the phenomena of color 

 vision generally accept one of two theories, or their modifications, in ex- 

 planation of the facts. 



The Young- Helmholtz Theory of Color Vision. This theory assumes 

 that there are three fundamental sensory elements in the retina which cor- 

 respond to and are stimulated primarily by the three primary colors red, 

 green, and violet. The theory in its present form further assumes that each 

 color-perceiving element is slightly stimulated by others of the spectral rays, 

 as shown in figure 474. When red rays fall upon the retina, they stimulate 

 the red-perceiving elements strongly and the green and violet very feebly. 

 The resulting sensation is that of red. So also is it with green and violet rays. 

 When the retina is stimulated by both red and green rays, the two correspond- 

 ing color-perceiving elements are strongly stimulated. The resulting color 

 perception, however, is a combination of the two sensations and corresponds 

 to some region of the spectrum between the red and green, according to the 

 relative intensity of the two stimuli. When all three color-perceiving ele- 

 ments are stimulated at the same time, the theory assumes that white light 

 will be perceived. In a similar manner all the various color sensations are 

 arrived at. 



