122 REFRACTION 1 



candle. The human eye possesses a most wonderful lens and 

 screen (Fig. 78); 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 crystal- 

 line lens /,, where they are refracted, 

 and then pass onward to the retina 

 R y where they form a distinct image 

 of the object. . 



We learned in Section 1 14 that a 

 change in the position of the object 

 necessitated a change in the position 

 FIG. 78. The eye. 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 located directly back of the iris. How, 

 then, does it happen that we can see clearly both near and 

 distant objects; that the printed page which is held in the 

 hand is visible at one second, and that the church spire on 

 the distant horizon is visible the instant the eyes are raised 

 from the book ? How is it possible to obtain on an immov- 

 able screen by means of a simple lens two distinct images of 

 objects at widely varying distances? 



The answer to these questions is that the crystalline lens 

 changes shape according to need. The lens is attached to 

 the eye by means of small muscles, m, and it is by the action 

 of these muscles that the lens is able to become small arid thick, 

 or large and thin; that is, to become more or less curved. 

 When we look at near objects, the muscles act in such a way 

 that the lens bulges out, and becomes thick in the middle and 

 of the right curvature to focus the near object upon the 



