18 THE OPTICAL DEFECTS OF THE EYE. 



tinctness. They can read the smallest type, when brought near the 

 eyes, even better than persons with normal vision, but they are not 

 able to recognize their friends at a distance of fifteen or twenty feet. 



In order to enable such persons to see distinctly at a distance, it is 

 necessary for them to wear concave spectacles of such a strength, that 

 the parallel rays from distant objects may have such a degree of diver- 

 gence, that, falling upon the myopic eye, they may form a focus upon 

 the retina. Theoretically, we should prescribe concave glasses of 

 such a strength that their focus will correspond with the patient's 

 " far " point. Thus, if tbe " far " point be at 1 2 inches, we should 

 prescribe — 12, as a twelve inch concave lens, placed before such an 

 eye, will give parallel rays from distant objects the same degree of 

 divergence as if they proceeded from the "far" point of the eye ; 

 namely, at 12 inches from the eye. Thus, in Fig. 9, P. P. represent 

 parallel rays falling upon the concave lens, A. B. ; they are n^ade to 

 diverge, as if coming from the focus, C, and falling upon the eye 

 divergingly, they are focussed upon the retina at F. Practically, how- 

 ever, we would find that — ] 2 would be rather too strr mg, and that — 

 25 or — 16 would probably answer better. As a rule, the weakest 

 glasses should be worn that will enable the patient to see distant 

 objects with distinctness. 



In testing the degree of myopia, we use a series of test types that 

 are so constructed that No. I (smallest) can be distinctly seen and read 

 by a person having normal vision, at a distance of 1 foot ; No. 11, at 

 2 feet ; No. V, at 5 feet ; No. XX, at 20 feet ; and so on. A speci- 

 men of these types will be annexed to this paper. The tvpes are 

 also used in testing the acuteness of vision in Presbyopia, Hyperme- 

 tropia, Amblyopia, &c. 



2. In determining the degree of myopia in any case, we ascertain 

 the greatest distance at which No. I test types can be read distinctly ; 

 if at 10 inches, the " far " point will be at 10 inches, and the myopia 

 would be called -^-^ ; if at 6 inches, the myopia would be called -}. 

 From this we can, as stated above, get a proximate knowledge of the 

 strength of the concave lens necessary to relieve the myopia. 



3. A myopic eye, when in a state of rest, is adjusted for diverging 

 ray». To enable such an eye to see distant objects, that is, to bring 

 parallel raya to a focus on the retina, it is necessary to give these 

 parallel rays a preliminary degree of divergence by the interposition 

 of the proper concave lens. 



