992 THE SENSES 



sufficiently well illuminated, sends rays out in all directions. 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 



Fig. 4 io. which the eye of the ob- 



server looks (Fig . 411)- 

 But the illumination thus obtained is comparatively faint ; and a 

 concave mirror is now 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. 413), the mirror 

 is held close to the observed eye, and an erect virtual image of 



Fig. 411. Figure 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 Helm- 

 holtz'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 unaccom- 

 modated) observing eye E. 



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 



