384 



COORDINATION AND SENSATION 



in the imperfect vision of objects, but they throw an extra 

 strain upon the nervous system and may render the pro- 

 cess of seeing exceedingly painful. 



A normal eye is able, when relaxed, to focus light accu- 

 rately from objects which are twenty feet or more away 

 and to accommodate itself to objects as near as five inches. 



An eye is said to be myopic, or 

 short-sighted, when it is unable 

 to focus light waves from dis- 

 tant objects, but can only dis- 

 tinguish the objects which are 

 near at hand. In such an eye 

 the ball is too long for the con- 

 verging power of the lenses, 

 and the image is formed in front 

 of the retina (C, Fig. 164). 

 A long-sighted, or hyper- 



metropic, eye is one which can 

 FIG. 164. Diagrams illustrat- .. ,. . f ,. .. 



ing long-sightedness and short- f CUS h S ht fr m dlstant ob J GCtS ' 

 sightedness, and method of but not from near objects. In 



remedying these defects by lenses, such an eye the ball IS too short 

 A. Normal eye. B. Long-sighted f Qr the conver gi ng power o f the 

 eye. C. Short-sighted eye. i i 



lenses and the image tends to 



form back of the retina (B, Fig. 164). These defects in 

 focusing are remedied by wearing glasses with lenses so 

 shaped as to counteract them. Short-sightedness is cor- 

 rected by concave lenses and long-sightedness by convex 

 lenses, as shown in diagrams above. 



Astigmatism is another defect in the focusing power of 

 the eye. In astigmatism the parts of the eye fail to form 

 the image in the same plane, so that all portions of the 

 object do not appear equally distinct. Certain parts of it 

 are indistinct, or blurred. The cause is found in some 



