THE SENSES 9i<> 



placed before it in order that the fundus may be distinctly 

 seen. 



When the observed eye is hypermetropic, the rays emerging 

 from the unaccommodated eye are divergent, and a convex 



Fie. 397.- -DIRECT METHOD OF USING THE OPHTHALMOSCOPE. 



Light falling on the perforated concave mirror M passes into the observed eye 

 E' ; and, both E' and the observing eye E being supposed emmetropic and unac- 

 commodated, an erect virtual image of the illuminated retina of E' is seen by E. 



lens, the strength of which is proportional to the amount of 

 hypermetropia, must be placed before the observer's unaccom- 

 modated eye if he is to see the fundus distinctly. By accommo- 



Fic. 398. USE OF THE OPHTHALMOSCOPE (DIRECT METHOD) FOR TESTING ERRORS 

 OF REFRACTION IN MYOPIC EYE. 



Rays issuing from a point of the retina of E', the observed (myopic and un- 

 accommodated) eye, pass out, not parallel, but convergent. They will therefore 

 be focussed in front of the retina of the observing (unaccommodated) eye E if 

 the latter is emmetropic. By introducing a concave lens L of suitable strength, 

 however, a clear view of the retina of E' will be obtained, and the strength of this 

 lens is the measure of the amount of myopia. 



dating, the observer can see the fundus clearly without a convex 

 lens. 



By this method errors of refraction in the eye may be detected 



