THE SENSE OF SIGHT. 293 



retina, and produce their effect on the rods and cones which constitute 

 the outer (back) layer; and the nerve impulses aroused by the light 

 must return through the thickness of the retina to be conveyed along 

 the nerve fibers of the innermost layer of the retina to the optic nerve. 



Importance of the Retina. The chief structure in the 

 eye is the retina. Without this all else is useless. If 

 light of sufficient strength falls on the retina, it stimulates 

 elements in the outer layer (rods and cones), and the nerve 

 impulses, thus started, pass along the fibers of the optic 

 nerve to the brain, and we have the sensation of sight. 

 But in order to see anything distinctly, the light must fall 

 on the retina in such a way as to form a distinct image of 

 that object. If the lens be removed, or becomes opaque, 

 as in " cataract," we fail to see distinctly, though we may 

 discern light from darkness. The other parts of the eye 

 exist to form images oh the retina. The cornea, lens, and 

 the aqueous and vitreous humors are the parts directly con- 

 cerned in forming the images. Light from an object passes 

 through the cornea, aqueous humor, lens, and vitreous 

 humor, and the rays are so refracted as to form an inverted 

 image. If this image falls on a good retina, we see well. 



The Blind Spot. The retina is much more complicated 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. Hence, this spot is 

 called the blind spot. 



Experiment illustrating the Blind Spot. 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 i, 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 



