148 Common Science 



the paper, naturally you again have a blur of light instead 

 of a point, and the image is not sharp and definite (Fig. 77). 



FIG. 77. Or if the light reaches the paper before it comes to a focus, the image 

 will be blurred. 



And that is why good cameras have the front part, 

 in which the lens is set, adjustable ; you can move the 

 lens back and forth until a sharp image is formed on the 

 plate. Motion-picture machines and stereopticons like- 

 wise have lenses that can be moved forward and back 

 until they form a sharp focus on the screen. Even the 

 lens in your eye has muscles that make it flatter and 

 rounder, so that it can make a clear image on the sensi- 

 tive retina in the back of your eye.' The lens in the 

 eyes of elderly people often becomes too hard to be 

 regulated in this way, and so they have to wear. one kind 

 of glasses to see things near them clearly and another 

 kind to see things far away. 



The kind of lens we have been talking about is the 

 convex lens. " Convex " means bulging out in the 

 middle. There are other kinds of lenses, some flat on 

 one side and bulging out on the other, some hollowed 



