660 THE SENSES 



duces a sensation of white light. But if the ray first passes through a prism, 

 then it produces sensations corresponding to the colors of the spectrum. As 

 is well known, white light is produced by vibrations of the luminiferous ether 

 through a wide range of vibration rates. When a beam of white light is 

 passed through a dispersing prism those vibration rates of low frequency 

 are refracted less than those of higher frequency, giving rise to the 

 spectrum. Vibrations of the luminiferous ether of rates just outside of the 

 spectral rates exist, those which have a lower rate giving rise to heat rays, 

 and those of higher rate to the so-called actinic or chemical rays, because 

 they exert a powerful chemical action. Those spectral colors which stimu- 

 late the retina to produce sensations of color presumably affect the retinal 

 elements through chemical changes which they produce there. But this 

 matter will be discussed under theories of color vision. 



The examination of color sensations reveals certain correspondences be- 

 tween the physical color of the stimulus and the resulting color perception. 

 If a pure spectral color be allowed to fall on the retina, a corresponding simple 

 sensation is produced. If two colors fall on the same portion of the retina 

 at the same time, a sensation is produced that is different from that which 

 occurs when either color alone stimulates. The same fact holds true for 

 three colors or more. In fact, three spectral colors can be selected which 

 by proper combination can be used to produce sensations of all the colors of 

 the spectrum. Such colors are called the fundamental colors, and while 

 the choice is more or less arbitrary, red, green, and violet are the colors usu- 

 ally considered. 



Extent of the Visual Field for Color. The retina is most sensitive 

 to color in the region of the macula lutea. If by means of the perimeter one 

 explores the retina to spectral red, for example, it is found that the color can 

 be identified only at a distance of from 30 to 50 from the macula; the 

 limits extending out somewhat farther on the nasal side of the retina, that 

 is, the part corresponding to the temporal visual field. In the same way yel- 

 low can be identified for from 40 to 70, blue from 40 to 50. The visual 

 field for green is quite restricted, usually extending only from 20 to 30. 

 The extent of the color visual field varies greatly in different individuals. 



Complemental Colors, and After-images of Color. Certain colors, 

 when allowed to stimulate the retina at the same time, tend to neutralize each 

 other. That is, they produce sensations approaching white, usually some 

 shade of gray, which will have a tinge of one or the other primary colors 

 according to the proportion of stimulation. These pairs of colors are called 

 complemental colors. Each spectral color has its complemental color, a fact 

 that is represented in figure 473. The complemental colors of greatest physi- 

 cal significance are red and green (greenish blue), yellow and deep blue 

 (indigo blue), green (greenish yellow), and violet. 



Positive after-images of color exist for a brief moment, but the greatest 



