EXPERIMENTS ON THE DIOPTRIC MECHANISM 121 



solution of atropine should previously be instilled into the eye. Or 

 a frog, with the body wrapped in a cloth, may be employed.] Only 

 a limited part of the retina is seen at one time, but it is much 

 magnified. 



The subject is seated in a darkened room with a light, not too 

 bright, near his ear. The observer sits in front of, and on a slightly 

 higher level than, the subject, close to him. The observer holds the 

 mirror in front of and close to his own eye, and, throwing the beam of 

 light into the subject's 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 (2 to 3 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 disc 

 (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 is enlarged because the 

 refracting surfaces of the subject's eye magnify the parts under 

 observation. 



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

 himself about 18 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 (2 to 3 inches focus) in his left hand, and, holding it 

 vertically between the thumb and forefinger at a distance of 2 to 3 

 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 disc 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 either the patient or observer 

 has abnormal vision this is corrected by suitable lenses placed behind 

 the aperture in the mirror. 



