246 



THE HUMAN MECHANISM 



regarded merely as a camera, is that in which distant objects 

 are focused on the retina when the muscle of accommoda- 

 tion is completely relaxed and the lens is thus flattened to 

 its utmost by the elastic pull of the choroid coat (p. 243), 

 for iii this case the eye is rested by looking at distant ob- 

 jects, and works only when looking at near objects. Such 

 an eye is known as an emmetropic eye (Fig. 97, E). 



Unfortunately, not all eyes meet this requirement. The 

 eyeball may be either too short or too long ; so that, with 

 the muscle of accommodation relaxed, the position of the 



.-A. 



FIG. 95. Action of a convex lens in bringing to a focus the rays of light 



diverging from a single point 

 The rays from A are focused at a ; those from B, at 6 



perfect focus for distant objects is either before or behind 

 the retina; the eye no longer sees distant objects distinctly 

 when it is at rest, because then the retinal image is blurred. 

 To understand more fully the undesirable consequences of 

 this condition, we must learn how convex lenses produce 

 images of objects. 



9. The action of a convex lens on rays of light. The rays 

 of light diverging from a single point and entering a convex 

 lens are bent so that all come together again in a point be- 

 hind the lens, or, as it is said, are brought to a focus. This 

 is shown in Fig. 95, as is also the fact that rays of light 

 diverging from more distant points come to a focus behind 

 the lens sooner than those diverging from nearer points. 



