THE SPECIAL SENSES 405 



Defects of the eye. There are several defects of the 

 eye which are common: short sight edness, far sight ed- 

 ness, and astigmatism. Spectacles are worn to compen- 

 sate for these defects. In a near sighted eye, objects 

 can be seen distinctly only when not more than about 

 ten inches away. If an object is at a greater distance, 

 the focus is formed in front of the retina. To remedy 

 this defect concave lenses are used. These will throw 

 the image farther back upon the retina. 



If the eye is far sighted, the image will fall behind 

 the retina unless the object is more than ten inches 

 away. To correct this convex glasses are used. These 

 Avill bring the focus farther forward. 



Astigmatism is a very common defect in the eye. 

 This is due to an irregular curvature in the surfaces 

 of the cornea or lens, or in both. The result is that 

 only a % part of the rays focus on the retina at once. 

 This produces a blurred image. The eye muscles 

 become strained in trying to adjust the lens to secure 

 a clear image. As a result headaches are frequently 

 produced. 



Care of the eyes. Too much care can not be taken 

 of the eye; first, because the eye is a very delicate 

 instrument, and second, because it is so very useful 

 to us. Frequently defects of the eye escape the atten- 

 tion. This is especially true in children. Nervousness 

 and headaches are often caused by defective vision. If 

 glasses, properly fitted by a competent oculist, are 

 worn, these troubles may be relieved. The light by 



