THE SEHSES. 



727 



after some practice, is the yellow spot, with the smaller lighter- colored fovea 

 centralis in its centre. This constitutes the direct method of examination (fig. 

 429) ; 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 ametropic, i.e., is myopic or 

 hypermetropic, 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 (fig. 430) 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' the 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, toward 

 or away from the face of the patient, until the out- 

 line 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 lens should be 

 kept fixed at a distance of two or three inches, the 

 mirror being alone moved until the disc becomes 

 visible : should the image of the mirror, however, 

 obscure the disc, the lens may be slightly tilted. 



Fig. 431. The ophthalmo- 

 scope. The small upper mir- 

 ror is for direct, the larger 

 for indirect illumination. 



Visual Purple. The method by which a ray of light is able to 

 stimulate the endings of the optic nerve in the retina in such a manner 

 that a visual sensation is perceived by the cerebrum is not yet under- 

 stood. It is supposed that the change effected by the agency of the 

 light which falls upon the retina is in fact a chemical alteration in the 

 protoplasm, and that this change stimulates the optic nerve-endings. 

 The discovery of a certain temporary reddish-purple pigmentation of the 

 outer limbs of the retinal rods in certain animals (e.g., frogs) which had 

 been killed in the dark, forming the so-called rJiodopsin or visual purple, 

 appeared likely to offer some explanation of the matter, especially as it 

 was also found that the pigmentation disappeared when the retina was 

 exposed to light, and reappeared when the light was removed, and also 

 that it underwent distinct changes of color when other than white 

 light was used. It was also found that if the operation were performed 

 quickly enough, the image of an object (optogram) might be fixed in the 

 pigment on the retina by soaking the retina of an animal, which has 

 been killed in the dark, in alum solution. 



