838 



THE EYE AND VISION 



[CH. LVIII. 



observer now approximates the mirror, with his eye to the eye of the patient, taking 

 care to keep the light fixed upon the pupil, so as not to lose the reflex. At 

 a certain point, which varies with different eyes, but is usually reached when 



there is an interval of about two or three inches 

 between the observed and the observing eye, the 

 vessels of the retina become visible. Examine 

 carefully the fundus of the eye, i.e., the red 

 surface until the optic disc is seen ; trace its 

 circular outline, and observe the small central 

 white spot, the porus opticus, or physiological 

 pit : near the centre is the central artery of the 

 retina breaking up upon the disc into branches ; 

 veins also are present, and correspond roughly 

 to the course of the arteries. Trace the vessels 

 over the disc on to the retina. Somewhat to 

 the outer side, and only visible after some 

 practice, is the yellow spot, with the smaller 

 lighter-coloured fovea centralis in its centre. 

 This constitutes the direct method of examina- 

 tion ; by it the various details of the fundus are 

 seen as they really exist, and it is this method 

 which should be adopted for ordinary use. 



If the observer is myopic or hypermetropic, 

 he will be unable to employ the direct method 

 of examination until he has remedied his de- 

 fective vision by the use of proper glasses. 



In the indirect method the patient is placed 

 as before, and the operator holds the mirror in 

 his right hand at a distance of twelve to eighteen 

 inches from the patient's right eye. At the same 

 time he rests his left little finger lightly upon the 

 patient's right temple, and holding a convex lens 

 between his thumb and forefinger, two or three 

 inches in front of the patient's eye, directs the 

 light through the lens into the eye. The red 

 reflex, and subsequently the white one, having 

 been gained, the operator slowly moves his 

 mirror, and with it his eye, towards or away 

 from the face of the 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 the fundus above described, but the image will be much smaller and in- 

 verted. The appearances seen are depicted in fig. 516. 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. 532 represents what occurs when employing the direct method. S is the 

 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, 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 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 w 1 respectively. 



Fig. 533 represents what occurs in examining the eye by the indirect method. 



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 focussed just behind 



FIG. 531. The Ophthalmoscope. The 

 small upper mirror is for direct, the 

 larger for indirect, illumination. 



