138 PHYSIOLOGY AND TEMPERANCE. 



43, the direction of the rays from the several points on their 

 way to the retina will be clearly seen. 



There is a delicate little muscle in the interior of the eye- 

 ball, attached to the membrane, which encloses and holds the 

 lens in place. Its use is important. It finds the focus for 

 varying distances, just as a boy pulls his spy-glass in or out 

 to get a correct focus according to the distance. Our eyes 

 are arranged naturally for seeing distant objects. When we 

 look at near objects, this little muscle is brought into action. 

 It draws upon the membrane and slackens it, allowing the 

 lens to become more rounded. The lens is rapidly adjusted 



FIQ. 43. Diagram showing how the image of an object is formed upon 

 the retina. 



in this way for varying distances, and brings the rays of light 

 to a focus exactly on the retina. If the lens be too flat, the 

 rays will not come to a focus before reaching the retina. If 

 it be too rounded, the rays come to a focus in front of the 

 retina. In either case the eyesight is defective. In the one 

 case the person will be far-sighted, and in the other near- 

 sighted. The far-sighted person finds relief in convex glasses, 

 the near-sighted person in concave glasses. 



It may not be the fault of the lens that the rays are not 

 brought to a focus on the retina. The eye-ball itself may 



