EXPERIMENTS ON THE DIOPTRIC MECHANISM 105 



eye, asks him to look upwards and inwards. The observer then moves the 

 mirror, with his eye close behind it, backwards and forwards, looking through 

 the hole in the centre, and when the proper distance is found (two to three 

 inches), the retina comes into view with its vessels running in different directions 

 on a red ground. The mirror 'is moved about until the optic disk (entrance of 

 optic nerve) is seen as a whitish circular area with the central artery and vein 

 of the retina emerging at its centre. The image of the fundus is virtual 

 (erect), and it is enlarged because the lenses of the subject's eye magnify the 

 parts behind. 



Indirect method. With the subject as before, the observer places himself 

 about eighteen inches in front of the patient and throws the light on the pupil 

 as in the direct method. He then takes a small biconvex lens (two to three 

 inches focus) in his left hand, and, holding it vertically between the thumb and 

 forefinger at a distance of two to three inches from the patient's eye, moves his 

 own eye with the mirror in front of it backwards and forwards and from side to 

 side until the optic disk and other parts of the retina are seen. The image is 

 real (inverted), and is only slightly magnified. 



Retinoscopy is difficult in the human subject unless the pupil has been 

 previously dilated by atropine. If the patient or observer has abnormal vision 

 this is corrected by suitable lenses placed behind the aperture in the mirror. 



