8oo A MANUAL OF PHYSIOLOGY 



can be successively focussed. The condition may be cor 

 reeled by glasses which are segments of cylinders cut 

 parallel to the axis. 



The Ophthalmoscope. The pupil of the normal eye is dark, 

 and the interior of the eye invisible, without special means 

 of illuminating it. But this is not because all the light that 

 falls upon the fun'dus is absorbed by the pigment of the 

 choroid, for even the pupil of an albino appears dark when 

 the eye is covered by a piece of black cloth with a hole in 

 front of the pupil. The explanation is as follows : 



Let the rays from a luminous point P be focussed by the 

 lens L at P' (Fig. 300). It is plain that rays proceeding from 



P' will exactly retrace 

 the path of those from 

 P and be focussed at P. 

 Now, the eye receives 

 rays from all directions, 

 and, when it is suf- 

 ficiently well illumi- 

 nated, sends rays out 

 FIG. 300. j n a ii 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 ophthalmo- 

 scopic 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. 301). 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. 302), 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 



