772 



THE BYE AND VISION. 



[CH. LV. 



of the patient. A red glare, known as the reflex, is seen ; it is due to the 

 illumination of the retina. The patient is then told to look at the little 

 finger of the observer's right hand as he holds the mirror ; to effect this the 

 eye is rotated somewhat inwards, and at the same time the reflex changes 

 from red to a lighter colour, owing to the reflection from the optic disc. The 

 observer now approximates the mirror, and 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 will 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 prac- 

 tice, is the yellow spot, with the smaller 

 lighter-coloured forea centralig in its 

 centre. This constitutes the direct method 

 of examination ; 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 hyperme- 

 tropic, he will be unable to employ the 

 direct method of examination until he 

 has remedied his defective 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 

 right temple, and holding a convex lens be- 

 tween his thumb and f orefinger,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 inverted. The appear- 

 ances seen arc depicted in fig. 572. 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. 587 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 



Fig. 586. The ophthalmoscope. The 

 small upper mirror is for direct, 

 the larger for indirect illumina- 

 tion. 



