THE OPHTHALMOSCOPE 



223 



slowly moved a little nearer to or further from the observed eye, and at the 

 same time an attempt made to accommodate the eyes for a point between 

 the observer and the lens. 



Fig. 174 gives diagrammatically the course of the light in this instance. 

 E is the observed eye and m m the concave mirror with its central aperture, 



Fig. 174.— The Course of the Light in the Indirect Method of Employing 

 the Ophthalmoscope. 



s a source of light the rays from which falling upon the mirror are reflected 

 to form an image at 0. They then diverge, but are again condensed by the 

 lens, and entering the eye, E, form a second image just behind the lens ; they 

 then again diverge and diffusely illuminate the fundus oculi. The rays of 

 light reflected from two points, i and m, on the fundus, diverging from the eye, 

 are refracted by the lens to form an inverted real image, i l m l , larger than 

 the object, i m. These latter rays then diverging are collected and focussed 

 by the observing eye e 1 to form an image i 1 >ir on the retina. 



The indirect method of examination is most generally useful because it 

 gives a large field of view under a low magnifying power (about five 

 diameters). In the view obtained it must be remembered that all the parts 

 are inverted, that seen to the right being from the left part of the fundus, 

 and vice versa. 



Fig. 175.— The Course of the Light in Examining the Eve r.v the 

 Direct Method. 



II. The direct method.— In this method the examination is made 'with 

 the mirror alone, without the intervention of the biconvex lens. In tins 

 method a small concave mirror is used, and is broujjhl as close as possible 

 to the observed eye, which should be accommodated For distant vision. The 



