COLOR VISION. 311 



that cause sensations 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. 



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. In 

 many of the lower animals, a fish, for instance, the two optic 

 nerves cross without any intermingling of fibers. 



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). 



Regulation of the Amount of Light Admitted into the 

 Eye. Hold a hand-glass between the face and a well-lighted 

 window. Note the size of the pupils. Quickly turn toward 

 the darkest part of the room. We see, what we have all 

 noticed in watching the eyes of a cat, that when subject to a 

 bright light the pupil is small, but with less light the pupil is 

 larger. The iris has circular muscle fibers that reduce the 

 pupil when .there is too much light for the eye, and when 

 the light is feeble the pupil opens wider. 



Color Sensations. The difference in colors is due to the 

 differences in the rapidity of the vibrations of the waves 

 of light, as in sound differences in the rapidity of the vibra- 

 tions of the sound waves cause the various degrees of pitch. 

 Many interesting experiments may be made with color sensa- 

 tion, 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 quickly 

 look at a point in the white surface. What color appears ? 

 This may be repeated with other colors. 



Color Blindness. It is found that some persons cannot 

 distinguish certain colors. Blindness to red and green are 



