THEORIES OF COLOR VISION 697 



upon a bright violet, and a violet tint upon a bright yellow surface. The 

 color excited thus must arise as an opposite or antagonistic condition of the 

 retina, and the opposite conditions of which it thus becomes the subject, 

 would seem to balance each other by their reciprocal reaction. A necessary 

 condition for the production of the contrast colors is that the part of the 

 retina in which the new color is to be excited shall be in a state of compara- 

 tive repose; hence the small object itself must be gray. A second condition 

 is that the color of the surrounding surface shall be very bright. 



Color-blindness. Many persons are unable to distinguish one or 

 more of the fundamental colors, and therefore have different perceptions 

 of the color combination from that of the normal individual. It is said that 

 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, 



