308 



FOR DISTANCE. 



Fig. 91. The Formation of an Image on the Retina. 



in front of a window, and catch the inverted image of the 

 window on a piece of paper held back of the lens. This illus- 

 trates how the image of an external object is formed by the 



crystalline lens upon 

 the retina of the eye. 

 If two lenses of dif- 

 ferent thickness can 

 be obtained, it will be 



86611 that the thicker 



lens (if both have the 



same diameter) will make an image closer to the lens than 

 the thinner one. 



Adjustment of the Lens for Seeing at Different Dis- 

 tances. If we look up from a book we are reading, we do 

 not realize that any change is necessary in the eye for us to 

 see a distant object. But it is easy to prove that we cannot, 

 at the same time, see distinctly a near and a distant object. 



Stick a pin at each end of a book cover. Hold the book at 

 about the usual distance for reading, so that the two pins are 

 in a line with the eye. Look closely at the nearer pin, and 

 the second pin will appear indistinct. Now look closely at 

 the head of the farther pin. The nearer one may be seen, but 

 not sharply. Another way of testing this : hold the tip of a 

 pencil in a line with any object, say a picture, on a wall oppo- 

 site. In looking at the tip of the pencil the picture is dim. 

 Now look by the pencil at the picture, and the point of the 

 pencil will be blurred. 



What changes does the eye make to enable it to see 

 clearly, now a near, now a far object? 



When the photographer places his camera, he moves the 

 ground-glass plate back and forth till the image is distinctly 

 formed on the plate. We cannot move the retina back and 

 forth, so we change the shape of the lens. When we look at 



