ORGANS OF SPECIAL SENSE THE EYE 



353 



FIG. 177. DIAGRAM 



Showing a nearsighted eye with a proper 

 lens for correcting the defect. 



glasses. The reason that the nearsighted person finds it hard to 

 see distant objects distinctly is that the rays forming the image 

 come to a focus too quickly after entering the eye; if lenses of 

 just the right curvature be 

 placed in front of his eyes, so 

 that the rays are caused to 

 diverge a little before reach- 

 ing the eyes (Fig. 177, dotted 

 lines), the rays will be brought 

 to a focus at exactly the right 

 place. By "fitting a person 

 with glasses" we simply mean 

 that extra lenses are chosen 

 of just the right curvature to correct his particular trouble. 

 Myopia is common among those who read very much or 

 use their eyes for other work at close range. To avoid devel- 

 oping nearsightedness, one should acquire the habit of hold- 

 ing books at least eighteen inches from the eyes. 



In farsightedness, or hyperopia, the conditions are just the 

 reverse of those in nearsightedness. The person cannot see a 

 near-by object clearly because 

 the lens is too flat or the eye- 

 ball too short. The rays of 

 light going into the eye come 

 together behind the retina, as 

 shown in Figure 178. In such 

 an eye even the full contraction 

 of the ciliary muscle may be 

 unable to make the lens convex 

 enough to bend the light rays 

 to a point by the time they reach 



the retina, and consequently the image is blurred. If, how- 

 ever, a slightly convex spectacle lens be used, which will 

 turn the rays and start them to a point, the lens of the eye 

 can do the rest; Fig. 178. 



FIG. 178. DIAGRAM 



Showing a farsighted eye with proper 

 correcting lens. 



