THE SENSE OF VISION 



641 



Two methods are usually adopted of using the ophthalmoscope, 

 one being known as the direct, the other as the indirect. In the first 

 case, there is obtained an erect view of a small area of the retina, 

 magnified about thirteen times; in the second case, a less magnified 

 and inverted view is obtained of a larger area of the retina. 



Fro. 371. EXAMINATION OF THE EYE OF A RABBIT AFTER ATK OPINE BY THE 

 INDIRECT METHOD. 



In the direct method, the source of light is placed at the side of 

 the head of the subject, so that no light falls directly on the cornea. 

 The mirror, which is somewhat strongly concave, is held a few inches 

 from the subject's eye, and is so tilted that light is directed into the 

 pupil. The observer uses his left eye to examine the subject's left 

 eye, and similarly his right eye for the subject's right eye. 



FIG. 372. To SHOW THE COURSE OF THE KAYS OF LIGHT FROM THE LAMP TO THKT 

 OBSERVER'S EYE BY THE INDIRECT METHOD. 



a, Lamp; 5, the fold of lens; c, lens; d, mirror of ophthalmoscope. 



In the indirect method, a somewhat larger but less concave or 

 a plane mirror is used. The mirror is held at a distance of about 

 18 inches, and if the accommodating power of the subject is intact 



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