310 THE HLIND SPOT. 



short, and need special glasses. The refracting surfaces (cor- 

 nea or lens) may be unequally curved, causing astigmatism. 

 For most of these defects the occulist will supply suitable 

 glasses. 



The Blind Spot. The structure of the retina is exceed- 

 ingly complicated, much more so than any of the other nerve 

 endings. Light must fall on these special structures to have 

 any effect. Falling on the optic nerve itself has no effect in 

 giving a sensation of light. And if the light falls on the 

 spot where the optic nerve enters the eyeball we see nothing. 

 At the left (as looked at by the class) of a long blackboard 

 make a bright circular spot, three inches in diameter, with 

 white or yellow crayon. Beginning at the right of this, write 

 the figures 1, 2, 3, etc., along the whole length of the board, 

 about eight inches apart. Let each pupil close the right eye 

 and look at the bright spot. Then let each read the figures, 

 passing slowly from one to another in order, at the same time 

 noticing whether the bright spot can be seen. To succeed in 

 this the eye must not be allowed to waver. Have the pupils 

 tell when the bright spot disappears, then read on, and note 

 when the spot reappears. 



In the following 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, while capable of carrying nerve impulses 



