VISION 



993 



a point of the retina leave the eye, even when it is unaccommo- 

 dated, as a convergent pencil; and the emmetropic non-accom- 

 modated eye of the observer must have a concave lens placed 

 before it in order that the fundus may be distinctly seen. 



When the observed eye is hyper- 

 metropic, the rays emerging from the 

 unaccommodated eye are divergent, 

 and a convex lens, the strength of 

 which is proportional to the amount 

 of hypermetropia, must be placed 

 before the observer's unaccommo- 

 dated eye if he is to see the fundus 

 distinctly. By accommodating, the 

 observer can see the fundus clearly 

 without a convex lens. 



By this method errors of refraction 

 in the eye may be detected and 

 measured. The observer must always 

 keep his eye unaccommodated, and 

 if it is not emmetropic, he must 

 know the amount of his short- or 

 long-sightedness i.e., the strength 

 and sign of the lens needed to correct 

 his defect of refraction, and must 



allow for this in calculating the defect of his patient. Non- 

 accommodation of the eye of the latter can always be secured by 

 the use of atr opine. 



Fig. 412. May's Electric Ophthal 

 rnoscope. 



Fig. 413. Direct Method of using the Ophthalmoscope. Light falling on the per 

 forated concave mirror M passes into the observed eye E' ; and, both E' and the 

 observing eye E being supposed emrnetropic and unaccommodated, an erect 

 virtual image of the illuminated retina of E' is seen by E. 



By the direct method of ophthalmoscopic examination, only a 

 small portion of the retina can be seen at a time, and this is highly 

 magnified. A larger, though less magnified, view can be got by 

 the indirect method. The observed eye is illuminated as before, 



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