466 THE SPECIAL SENSES 



The sensitiveness of the retina is readily exhausted. Hence, if 

 we look at a bright light for a short time that portion of the retina 

 upon which the image falls soon becomes exhausted and loses its 

 power to transmit impulses. If, now, we tm-n the eye from the 

 light to a white surface, there appears to be a black spot, on it, due 

 to the rays which fall upon the blinded spot, in the retina. 



Color sensations are due mainly to a difference in the quality 

 of the impulses, and these differences are due to different rates 

 of vibration, each color having its own time of vibration. White 

 light is a mixture of the impulses of all colors, blackness the 

 absence of all light impulses There are also many different 

 shades of color in sensation which are produced by combining 

 impulses from two primary colors Thus, green is a mixture of 

 yellow and blue. Lack of ability to distinguish colors is called 

 color blindness. In persons thus afflicted the light waves of 

 several different colors may produce the same sensation in the 

 retina. This affliction is due to a defect in the cones of the 

 retina. 



In binocular vision, images of the object are projected on both 

 the retinas, but the impulses thus set up and transmitted by the 

 two sets of nerve fibers give us only a single sensation in the brain. 

 This shows that the sensation of sight is not a simple action but 

 the result of association and fusing of impulses in the brain 

 cells. 



The intensity of the illumination increases the intensity of the 

 stimulus and determines the brightness of the sensation. Very 

 strong light or very dim light tends to strain the eye and should 

 be avoided. Individuals differ among themselves and from the 

 lower animals in the amount of light necessary to arouse sensa- 

 tion. Cats, for example, are able to distinguish objects with an 

 illumination far less than that required by human eyes. Certain 

 animals, such as the owls, whose eyes respond to the weak light of 

 the night, become almost blind in the bright light of day. The 

 retinal end organs may be stimulated by other means than by 

 light waves. Blows on the eyes or head may produce sensations 

 of light, and we " see stars," as the saying goes. The shadows 

 cast by blood corpuscles of the eye blood vessels often modify the 

 effects of vision. 



