THE OPTICAL DEFECTS OF THE EYE. 27 



tival suture, by which means we have the power of regulating our 

 operation, in proportion to the effect we wish to produce. 



When Strabismus shows itself in childhood, it should be treated 

 without delay, for, if not corrected, the vision of the " cross-eye " will 

 very soon become impaired. 



To get the full benefit of spectacles, in cases of hypermetropia, 

 they should be used both on the street, and at church, as well as 

 when reading or writing, — in fact whenever the eyes are used. 



The characteristics of a hypermetropic eye then are : 

 1st. Parallel rays form a focus behind the retina. 

 2nd. The " far " point is at an definite distance and negative. 

 3rd. The eye, in a state of rost, is adjusted for convergent rays. 

 4th. Convex glasses improve vision. 



5th. This affection is usually accompanied by symptoms of Asthe- 

 nopia and Amblyopia, and frequently by convergent strabismus. 



Chapter V. — Presbyopia. 



This affection usually develops itself between the ages of 40 and 

 45. Most persons at this age, although previously enjoying excellent 

 vision, complain that their sight, particularly in the evening, is 

 beginning to fail for near objects, as small print, &c., although they 

 can see distant objects as well as ever. 



In reading they will hold the book or paper at nearly arm's length 

 and perhaps bring the lamp almost between their eyes and the page. 

 Reading in this manner soon fatigues them, and they are obliged 

 frequently to rest, — or to resort to spectacles. 



In childhood, vrhen the vision is normal, the "near" point is 

 from S^ to. 4 inches from the eye, and the "far" point at an unlim- 

 ited distance ; that is, we can see objects distinctly as near as from 

 3^ to 4 inches from the eye, and we can see objects clearly (the size 

 being in proportion to the distance) from that to an indefinite dis- 

 tance. As age advances the "near" point recedes. At the age of 

 40 the " near " point is about eight inches from the eyes. When the 

 "near" point recedes to a greater distance than 8 inches, Donders 

 calls it a case of presbyopia ; Laurence, however, thinks that it 

 should not be called presbyopia unless the "near" point is at least 

 10 inches from the eye. 



