THE OPTICAL DEFECTS OF THE EYE. 23 



When a person with hypermetropia, attempts to read or write, 

 t>r accommodate his eyes to short distances, it is necessary for him to 

 tax his accommodation to its utmost extent, in order to bring the 

 diverging rays to a focus on the retina. This excessive effort at ac- 

 commodating the eye for short distances, can not be kept up for more 

 than a few minutes, when the ciHary muscle begins to relax, — the 

 " near " point commences to receed, and (if he is reading) the letters 

 become indistinct. The eye also feels fatigued, and other symptoms 

 arise which will be referred to when speaking of Asthenopia. 



Diagnosis. — When we suspect a patient has hypermetropia, we test 

 his eyes as follows : — We place a series of test-types, No. xv., xx., 

 XXX., &c., at a distance of about 20 feet. If he can read No. xv. or 

 XX. at this distance, his acuteness of vision is normal. We then try 

 his vision with weak convex glasses, say No. 50, and if he can read the 

 same type, at the same distance, we try successively No. 40, 36, 30, 24, 

 &c., until we reach the glasses that render the test type indistinct at that 

 distance. Some persons may possibly be able to relax their accommo- 

 dation so as to see as well at a distance, with convex 50 lenses, as without 

 them ; and not be hypermetropic ; it would, however, be very strong 

 presumptive evidence of its presence ; and if, in addition, the patient 

 complain of the symptoms of Asthenopia, we would be generally safe in 

 pronouncing it a case of hypermetropia. The shorter the focus of the 

 lens he can use, the stronger is the presumptive evidence of the disease. 



Again, if another patient be tested with the same type, at the same 

 distance, and we find that he can not read a smaller type than No. xl. 

 at 20 feet without spectacles, and that he can read No. xv. or xx. with 

 <Jonvex glasses, say + 10 or + 12, his would be called a case of 

 hypermetropia absolute. 



In order, however, to test accurately the degree o( hypermetropia 

 in any case, it is necessary to neutralize one element in the refractive 

 power of the eye ; namely, the power of accommodation. In most 

 cases of hypermetropia, particularly in young subjects, the accommo- 

 dation of the eye is so constantly exercised, even when directed to 

 distant objects, that it is quite impossible for them, by any effort of 

 their own, to completely relax that accommodation. I related in a 

 former chapter, the case of a patient who had lost the power of 

 accommodating his eye to different distances. As the refraction 

 of his eye was normal, parallel rays were brought to a focus upon the 

 retina, and vision for distant objects remained perfect. 



