294 PHYSIOLOGY. 



to waver. Have the pupils tell when the bright spot disappears, then 

 read on, and note when the soot reappears. 



Another Experiment. In this experiment shut the right eye, and 

 be careful not to let the left eye waver. 



Read this line slowly. Can you see the star all the time ? If the 

 star does not disappear before reaching the end of the line, let the eye 

 travel on across 'the right-hand page, or hold the book nearer the face. 

 In the human eye the optic nerve enters the eye not in the center, but 

 nearer the nose, so that in turning the left eye toward the right at the 

 proper angle, the image of the star falls upon the spot where the optic 

 nerve enters. As this spot is insensitive to light, the star no longer 

 appears. 



The Optic Nerve not Sensitive. The optic nerve, 

 while capable of carrying nerve impulses that cause sensa- 

 tions of light, is not itself sensitive to light. If the optic 

 nerve be cut, it does not give pain, but gives the sensation 

 of a flash of light. 



Sympathy between the Two Eyes. While most of 

 the fibers from each optic nerve cross to the other side of 

 the brain, some fibers go to the same side of the brain. 

 We can therefore better understand the close sympathy 

 that we know exists between the two eyes. 



Pain in the Eyes. Pain, felt in the eyes, comes from 

 impulses conveyed, not by the optic nerve, but by a branch 

 of the fifth pair of nerves (the nerves of sensation for most 

 of the face). 



Color Sensations. The difference in colors is due to the differ- 

 ences in the rapidity of the vibrations of the waves of light, as in sound 

 differences in the rapidity of the vibrations of the sound waves cause 

 the various degrees of pitch. Many interesting experiments may be 

 made with color sensation, most of which are difficult of explanation. 

 Fasten a bright red wafer or seal on a white card. Look intently at 

 the center of the red spot till the eye is tired. Then quickiy look at 

 a point in the white surface. What color appears ? This may be 

 repeated with other colors. 



