CH. LV.] 



THE OPHTHALMOSCOPE. 



773 



examined, to a point in the vitreous humour, and this produces a diffuse 

 lighting of the interior of the eyeball. Rays of light issuing from the point 

 p emerge from the eye parallel to one another, and enter the observer's eye 

 E 1 ; they are brought to a focus />' on the retina as the eye is accommodated 

 for distant vision. Similarly the point m and n will give rise to images at 

 w 1 and ' respectively. 



Fig. 588 represents what occurs in examining the eye by the indirect method. 



E 1 



M 



Fig. 587. The course of the light in examining the eye by the direct method. 

 (T. G. Brodie.) 



S is the source of light, M M the mirror, E the observed, and E 1 the 

 observing eye as before. The rays of light are reflected from the mirror 

 and form an image at u l ; they then diverge and are again made convergent 

 by the lens L held in front of the eye by the observer ; by this means a 



Fig. $88. The coune of the light in examining the eye by the indirect method. 

 (T. G. Brodie.) 



second image is focusscd just behind the crystalline lens of the eye E. They 

 the n again diverge and diffusely light up the interior of the eyeball. The 

 rays of light reflected from two points i and m on the retina diverging 

 from the eye are refracted to the glass lens L, and give an inverted real 

 image ' /' larger than the object i m. These latter rays then diverge, and 

 are collected and focussed by the observing eye E 1 to give an image i* m 3 on 

 the retina. (T. G. Brodie.) 



