THE SENSE OF VISION 



617 



metre, a 3-diopter (3 D.) lens one of ^ metre, and so on. Biconvex 

 lenses are called + , biconcave - , lenses. 



Retinoscopy, or Skiascopy affords an accurate method of testing 

 the refraction of the eye. The direction is observed in which the 

 " shadow " in the eye moves when a light is reflected into it from a 

 mirror. The " shadow " is seen against the illuminated background 

 of the eye, which appears a brilliant red choroidal reflex, as it is 

 called. The operation takes place in a darkened room, preferably 

 on an eye with the pupil dilated by a drug, such as homatropine. 

 From a metre distance the eye is observed through a small plane or 

 concave mirror with a hole in the centre the retinoscope. The 



FIG. 348. DIAGRAM SHOWING A, COURSE OF PARALLEL RAYS IN MYOPIC EYEJ 

 B, PATH OF PARALLEL RAYS IN A MYOPIC EYE AFTER CORRECTION BY MEANS 

 OF A BICONCAVE LENS. 



mirror is then slowly tilted from side to side. When a plane mirror 

 is used, the shadow moves in the same direction as the mirror is tilted 

 in the normal and in the hypermetropic eye, in the opposite direction 

 in the myopic eye. 



If the eye be normal, a lens of 1 D. will bring about a reversal of 

 the direction. In the case of hypermetropic and myopic eyes, the 

 reversal of the image is ascertained by introducing respectively 

 biconvex and biconcave lenses ; - 1 D. is added to the lens, which 

 completely neutralizes the shadow: in other words, is subtracted 

 from the strength of the + lens used in the case of hypermetropia, 

 and added to the strength of the - lens employed in myopia. If a 

 concave mirror be used as the retinoscope, the shadow moves against 

 the direction in the normal eye in hypermetropia, and in the same 

 direction in myopia. 



