258 REFRACTION 



obtain a large-sized photograph ; by increasing the distance be- 

 tween the camera and the object, a smaller photograph is ob- 

 tained. The mountain top may be so far distant that in the 

 photograph it will not appear to be greater than a small stone. 

 Many familiar illustrations of lenses, or curved refracting 

 surfaces., and their work, are known to all of us. Fish globes 

 magnify the fish that swim within. Bottles can be so shaped 

 ihat they make the olives, pickles, or peaches that they 

 contain appear larger than they really are. The fruit in 

 bottles frequently seems too large to have gone through the 

 neck of the bottle. The deception is due to refraction, and the 

 material and shape of the bottle furnish a sufficient explanation. 

 By using lenses of various kinds, it is possible to produce an 

 image of almost any desired size, and in any desired position. 



The human eye. We have seen how an image of a candle 

 can be obtained on a movable screen, by means of a simple 



lens. The human eye possesses a 

 most wonderful lens and screen 

 (Fig. 131); the lens is called the 

 crystalline lens, and the screen is 

 called the retina. Rays of light pass 

 from the object through the pupil P, 

 go through the crystalline lens L, 

 where they are refracted, and then 

 pass onward to the retina R, where 



they form a distinct image of the 

 FIG. I3 i.-Theeye. object 



We have learned that a change in the position of the object 

 necessitated a change in the position of the screen, and that 

 every time the object was moved the position of the screen had 

 to be altered before a clear image of the object could be obtained. 

 The retina of the eye cannot be moved backward and forward, 

 as the screen was, and the crystalline lens is permanently 





