358 ADVANCED PHYSIOLOGY 



second; the other colors which we can see are caused by 

 vibrations at rates between those of red and violet. Although 

 these figures really mean very little to us, it is certainly im- 

 probable that the material particles which we call rods and 

 cones can be thrown into such rapid movement, and especially 

 hard is it to see how their rates of vibration could be changed 

 from one frequency to another with such accuracy as they 

 must be when one suddenly looks from red to violet, or from 

 yellow to green, for example. 



In spite of the reasons which make it seem hard to believe 

 the action of light on the retina of the eye to be either chemical 

 or mechanical, we can hardly avoid the conclusion that it 

 must be one of these two, or a combination of them. We are 

 practically forced to say that we do not know what light 

 really does in the eye. 



COLOR VISION AND COLOR BLINDNESS 



We have already noted that in the back of the eye cones 

 are especially abundant and diminish toward the sides of the 

 retinal area, while rods are few at the back and plenty on the 

 sides. Hence light entering the eye from the side would 

 strike mainly on rods, while that going straight toward the 

 back would disturb chiefly cones. If we keep this fact in 

 mind, and reflect that the color of an object is most plainly 

 seen when the object is straight in front of us, we must con- 

 clude that the cones are the more sensitive of the two to color 

 changes. Rods seem to be sensitive to light and shade, but 

 not to color, while cones are sensitive to both. It can be 

 proved by experiment, too, that a larger area of the retina 

 is sensitive to one color than to another; for example, one can 

 see violet objects as they are brought around slowly from the 

 side to the front sooner than he can identify a green object so 

 brought before him. Consequently we conclude that the area 

 of the eye which can see violet is larger than that which can 

 perceive green. The area which can see red is also larger than 



