352 PHYSIOLOGICAL PHYSICS, rchap. xxvin. 



placed at a distance from it equal to that of distinct 

 vision, that is, still farther from the observed eye. 

 But at this distance the field of vision is so extremely 

 small that nothing can be distinguished. Moreover, 

 the person in endeavouring to see this image interposes 

 himself between the source of light and the eye to be 

 observed, and so cuts off the verj- rays whose reflection 

 he wishes to intercept. In all circumstances, con- 

 sequently, the eye appears dark. If, however, an 

 observer throws light into the eye from a mirror, and 

 if he places his eye behind the mirror, through an 

 opening in which he can look, he does not intercept 

 the rays, and he can find the conjugate focus of the 

 rays reflected from the ocular chamber, and thus per- 

 ceive an image of the structures reflecting the light. 

 This was the method at first employed by Helmholtz. 

 He sat in front of a patient, at whose side was a 

 lamp. By means of a plate of glass held in front of 

 one of his eyes, and placed at an angle to the light, he 

 directed rays from the lamp into the person's eye 

 through the pupil. Some of the light is absorbed by 

 the eye, but some is reflected outwards, along the 

 same paths by which it reaches the eye, to the plate 

 of glass. Here again some of the rays are reflected ; 

 but some pass through the plate into the observer's 

 eye, and so there is perceived an image of the retina 

 and other deep parts. 



Instead of the plate of glass a slightly concave 

 mirror was afterwards substituted, which permits a 

 greater concentration of light through the pupil of 

 the observed eye. The concave mirror is pierced in 

 the centre by a small opening, through which the 

 observer looks. 



Now the ophthalmoscope may be used with or 

 without lenses. If the ophthalmoscope be used 

 without lenses, on illuminating the back of the eye to 

 be observed, and on the observer bringing his eye 



