296 EXPERIMENTAL PHYSIOLOGY. 



relax his accommodation 1.., look as it were at a distant 

 object, when the retina comes into view as an erect or up- 

 right object. 



(c.) Observe the retinal blood-vessels running in different 

 directions on a red ground. Move the mirror about to find 

 the optic disc, with the central artery emerging from it. 

 Trace the course of the veins accompanying the arteries 

 across the disc. 



2. The Indirect Method, giving an inverted image. 



(a.) The patient, the light, and the observer are as before. 

 The observer places himself about twenty to eighteen inches 

 from the patient, and, holding the mirror in his right hand, 

 by means of the mirror he throws a beam of light into the eye 

 of the patient. When the eye is illuminated, he takes a small 

 biconvex lens of two to three inches focus in his unemployed 

 hand the left in this case holding it between his thumb 

 and index finger, placing it vertically two or three inches 

 from the observed eye. To ensure that the lens is held 

 steadily, rest the little finger upon the temple or forehead of 

 the patient. Keep the lens steady, and move the mirror 

 until the optic disc is seen, with the details already de- 

 scribed. 



Both methods ought to be practised, as each has its advantages. 

 In the direct method only a small part of the retina is seen at 

 one time, but it is considerably magnified ; while by the indirect 

 method, although more of the retina is seen at once, it is magnified 

 only slightly. 



If the observed or observer's eye is abnormal, suitable glasses 

 to be fixed behind the mirror are supplied with every ophthalmo- 

 scope. In some forms of ophthalmoscope, such as that of Gowers 

 and others, these lenses (convex + , and concave ) are fixed to 

 a rotating disc behind the mirror. As the disc is rotated, lens 

 after lens can be brought to lie exactly behind the hole in the 

 mirror, and thus correct any anomaly of refraction. 



3. Eye of a Living Rabbit. 



(.) Instil atropin as before, or use an atropinised gelatin 



