SENSE ORGANS. 



DEFECTS OF THE EYE AS AN INSTRUMENT. 



113 



We have shown the beauty of the eye as an instru- 

 ment by comparing it with the photographic camera. 

 But all eyes are not perfect. The defects of the eye are 

 indeed quite common, and apparently becoming more 

 and more common through abuse of this delicate organ, 

 especially in the schoolroom. In order to understand 

 these defects it is necessary to define the normal eye. 



Normal Sight — Emmetropy. — The normal eye in 

 a passive state is prearranged for a perfect image of a dis- 

 tant object. The focus of parallel rays is on the retina. 

 For all nearer distances it accommodates itself by action 

 of the ciliary muscles until the object is as near as five 

 or six inches. Nearer than this it can not accommodate 

 itself to make a perfect image. Its range of distinct 

 vision, therefore, is from six inches to infinite distance. 

 This is the standard. Any considerable deviation from 

 this is a defect. The most common defects are myopy, 

 hyperopy, prcsbyopy, and astigmatism. 



Myopy, Brachyopy — Nearsightedness.— This is 

 perhaps the most common of all defects of the eye, espe- 

 cially in large cities and in most advanced communities. 

 In the myopic eye the refractive power of the lenses of 

 the eye is too great for the position of the retina. The 

 focus of parallel rays when the eye is passive is not 

 on the retina, but in front of it. The rays must be di- 

 vergent to make a perfect image on the retina. There- 

 fore distant objects can not be seen distinctly. The 

 object must be brought near to a certain limit before it 

 can be seen well. But within that limit it accommodates 

 itself like the normal eye. In the normal eye the range 

 of distinct vision is from infinite distance to six inches; 

 in the myopic eye the range is from a yard to four 

 inches, or a foot to three inches, or six inches to two 



