16 THE OPTICAL DEFECTS OF THE EYE. 



The " near " and "far " point. — The nearest point to which objects 

 can be brought to an eye and be seen with perfect distinctness, is 

 called the " near " point, and the farthest point of distinct vision is 

 called the " lar " point. 



In a normal eye the " near " point is about seven inches from the 

 front of the cornea, and the "far" point is at an unlimited distance. 

 In childhood, however, the " near " point is about 3|^ inches from the 

 rye and recedes as age advances. At the age of forty the "near" point 

 of a normal eye is nearly eight inches from the eye. 



When the " near " point recedes to a greater distance than eight 

 inches from the eye it becomes inconvenient ; such an eye is called 

 presbyopic or long-sighted. 



When the "far" point is not unlimited, but is at a defi^iite distance 

 from the eye, as for instance from six inches to four or five feet from 

 the eye — such an eye is called myopic or short-sighted. 



Hang e of Accommodation. — The distance between the "near" and 

 "far" point in any eye, is called the "range of accommodation." If 

 a person can read distinctly very fine type at four inches from the eye, 

 and can also see clearly at an infinite distance the range of accommo- 

 dation would be said to equal \ because, when such an eye is directed 

 to objects at an infinite distance, (accommodated for parallel rays) in 

 order to see clearly objects only four inches distant, it is necessary to 

 increase the curvature of the crystaline lens, or in other words the 

 "power" of the eye to an extent equal to the addition of a 4 inch con- 

 vex lens ; the power of which is expressed by \. If a person's "near" 

 point is at eight inches from the eye, and his "far" point at an 

 infinite distance, his range of accommodation would be said to equal \. 



If the "near" point of a myopic eye be 3 inches, and the " far" 

 point be 12 inches, we get the range of accommodation by the 

 eqiiation ^— tV^T' 



Chapter III. — Myopia. 



Concave Lenses. — Before proceeding to the consideration of 

 Myopia, it will be well for us to glance at some of the properties of 

 concave lenses ; and, in order to simplify the subject, we will confine 



1st. The pupil contracts ; 2nd. The pupillary edge of the iris moves forward ; 8rd. 

 The peripheral portion of the iris moves backwards; 4th. The anterior surface of 

 the lens becomes more convex (arched) ; 5th. The lens does not change its posi- 

 tion ; 6th. The cornea retains the same degree of curvature. 



