EXTENT OF THE VISUAL FIELD FOR COLOR 695 



possibly of the rods is also said to occur, as has been already incidentally 

 mentioned. Under the influence of the stimulus of light the outer parts of 

 the cones, which in an eye protected from light extend to the pigment layer, 

 are retracted. In is even thought by some that the contraction is under 

 the control of the nervous system. Finally, according to the careful re- 

 searches of Dewar and McKendrick, and of Holmgren, it appears that the 

 stimulus of light is able to produce an action current in the retina. Mc- 

 Kendrick believes that this is the electrical expression of those chemical 

 changes in the retina of which we have already spoken. 



Color Sensations. When a ray of sunlight enters the eye it produces 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 re- 

 fracted 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 stimulate 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 

 usually 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 special 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. 



