THE SENSES 803 



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 

 atropia. 



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, 



FIG. 305 INDIRECT METHOD OF USING THE OPHTHALMOSCOPE. 



The rays of light issuing from E', the observed eye, are focussed by the biconvex lens 

 L, and a real inverted image of a portion of the retina of E', magnified four or five 

 times, is formed in the air between the lens and the observing eye E. This image is 

 viewed by E at the ordinary distance of distinct vision (10 or 12 inches). (The exaggera- 

 tion of the size of the mirror makes it appear as if some of the rays from the lamp 

 passed through the lens before being reflected from the mirror. This would not be the 

 case in an actual observation. ) 



view can be got by the indirect method. The observed eye 

 is illuminated as before, but the mirror and the observer's 

 eye are at a greater distance (Fig. 305). Here the rays 

 from a considerable portion of the retina are brought to a 

 focus by a convex lens held near the eye of the patient, so as 

 to form a real and inverted aerial image of the retina. This 

 image is viewed by the observer at his ordinary visual dis- 

 tance. It is not necessary in this method that the observed 

 eye should be non-accommodated, although it is convenient 



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