794 



THE EYE AND VISION 



[GIT. LVI. 



patient, until the outline of one of the retinal vessels becomes visible, when very 

 slight movements on the part of the operator will suffice to bring into view the 

 details of thefundus above described, but the image will be much smaller and in- 

 verted. The appearances seen are depicted in fig. 574. The lens should be kept 

 fixed at a distance of two or three inches, the mirror alone being moved until the 

 disc becomes visible : should the image of the mirror, however, obscure the disc, the 

 lens may be slightly tilted. 



The two next figures show diagrammatically the course of the rays of light. 



Fig. 589 represents what occurs when employing the direct method. S is the 



M 



Fio. 589. The course of the light in examiningjthe_eye by the direct method. (T. G. Brodie.) 



source of light, and M M the concave mirror with its central aperture, which reflects 

 the rays ; these are focussed by the eye E, which is being examined, toapointin 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 p 1 on the retina as the 

 eye is accommodated for distant vision. Similarly the point m and n will give rise 

 to images at m 1 and n l respectively. 



Fig. 590 represents what occurs in examining the eye by the indirect method. 



M 



Fio. 590. The course of the light in examining the eye by the indirect 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 o 1 ; 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 second image is foc-ussed just behind 

 the crystalline lens of the eye E. They then again diverge and diffusely light up 



