9i8 A MANUAL OF PHYSIOLOGY 



P. Now, the eye receives rays from all directions, and, when 

 it is sufficiently well illuminated, sends rays out in all direc- 

 tions. The moment, however, that the observing eye is placed 

 in front of the observed eye, the latter ceases to receive light 

 from the part of the field occupied by the pupil of the former, 

 and therefore ceases to reflect light into it. 



This difficulty is avoided by the use of an ophthalmoscopic 

 mirror. The original, and theoretically the most perfect, form 

 of such a mirror is a plate, or several superposed plates, of glass, 

 from which a beam of light from a laterally placed candle or 

 lamp is reflected into the observed eye, and through which the 

 eye of the observer looks (Fig. 396). But the illumination thus 

 obtained is comparatively faint ; and a concave mirror is now 



FlG. 396. FlOURE TO ILLUSTRATE THE PRINCIPLE OF THE OPHTHALMOSCOPE. 



Rays of light from a point P are reflected by a glass plate M (several plates 

 together in Helmholtz's original form) into the observed eye E'. Their focus 

 would fall, as shown in the figure, at P', a little behind the retina of E. The 

 portion of the retina AB is therefore illuminated by diffusion circles ; and the 

 rays from a point of it F will, if E' is emmetropic and unaccommodated, issue 

 parallel from E' and be brought to a focus at F' on the retina of the (emmetropic 

 and unaccommodated) observing eye E. 



generally used. In the centre is a small hole or a small unsilvered 

 portion of the mirror for the observer's eye. In the direct 

 method of examination (Fig. 397), the mirror is held close to 

 the observed eye, and an erect virtual image of the fundus is 

 seen. When the eye of the observer and of the patient are both 

 emmetropic, and both eyes are unaccommodated, the rays of 

 light proceeding from a point of the retina of the observed eye 

 are rendered parallel by its dioptric media, and are again brought 

 to a focus on the observer's retina. 



If the observed eye is myopic, the rays of light coming from 

 a point of the retina leave the eye, even when it is unaccommo- 

 dated, as a convergent pencil ; and the emmetropic non- 

 accommodated eye of the observer must have a concave lens 



