THE SPECIAL SENSES. 213 



seen that are at a distance, and the condition of the vision 

 is known as "long-sight," or hyperopia. It will be ob- 

 served, by reference to Fig. 51, that the focus of the 

 rays of light would fall behind the retina of this eye. 

 When the globe is too long, objects can only be clearly seen 

 that are very near to the eye; and the condition resulting 

 from this defect is termed " short-sight," or myopia. The 

 focus of the rays of light is, in this case, formed in the 

 interior of the eye in front of the retina. 



81. Long-sight, or hyperopia, is common among school- 

 children, nearly as much so as short-sight, and must not be 

 confounded with the defect known as the " far sight" of 

 old people; although in both affections the sight is im- 

 proved by the use of convex glasses. Children not infre- 

 quently discover that they see much better when they 

 chance to put on the spectacles of old persons. For the 

 relief of short-sight, concave glasses should be employed; 

 as they so scatter the rays of light as to bring the focus to 

 the retina, and thus cause the vision of remote objects 

 to become at once distinct. That form of "squint," in 

 which the eyes are turned inward, is generally dependent 

 upon long-sight, while that rarer form, when they turn 

 outward, is due to short-sight. 



82. The Function of Accommodation. If, after 

 looking through an opera-glass at a very distant object, it 

 is desired to view" another nearer at hand, it will be found 

 impossible to obtain a clear vision of the second object un- 

 less the adjustment of the instrument is altered ; which is 

 effected by means of the screw. If an object, like the end of 

 a pencil, be held near the eye, in a line with another object 

 at the other side of the room, or out of the window, and 

 the eye be fixed first apon one and then upon the other, it 

 will be found that when the pencil is clearly seen, the fur- 

 si. Lone-sight, how common ? With what must it not be con ounded ? Kind 



of jrlasse* for efiort-fight ? Why ? Squint ? 



82. What is- stated in connection with the opera-glass? Experiment with 

 pencil and distant object ? 



