340 APPLIED PHYSIOLOGY 



bringing the object very near the eye. Such persons cannot see dis- 

 tinctly at a distance greater than a few inches, but walk about as in 

 a perpetual fog. 



Diagram of the eye in near sight. 



The lens b brings the rays from a point of light a together at c too soon. So 

 the rays cross and fall over the whole surface of the retina from d to e, making a 

 confused image instead of a clear point. When the rays are spread apart by 

 bringing the light near the eye, as at / they come together farther away upon the 

 other side of the lens, as at g. Thus they fall upon a single point of the retina 

 and produce a clear image. 



Near sight can be remedied by placing in front of the eye a lens 

 which will make the rays more diverging. So the spectacles have the 

 glasses hollowed out instead of bulging. 



618. Astigmatism and headaches. Sometimes the lens or 

 cornea is more curved in one direction than in another. Then a part 

 of the object will seem distinct, and another part blurred, and the eye 

 muscles will constantly change the focus in the attempt to obtain a full 

 and clear image. This is very tiresome to the eyes, and often causes 

 severe headaches. The remedy is to use a glass which is curved in one 

 direction only, so as to correct only the defective part of the lens. 



619. Cataract. Sometimes the lens becomes hard and white. 

 Then no light can pass through, and there is total blindness. This is 

 called a cataract. By a simple and safe operation the lens can be 

 removed, when the light will fall upon the retina as before. Spectacles 

 to take the place of the lens enable the person to see. 



620. Judgment of position. In perfect vision each eye 

 is turned toward the same point, so that the images fall 

 upon corresponding points of the retina. But the two eyes 

 view an object from different positions, and so the images 



