THE SENSE OF VISION 



615 



mere spot, and inverted, from the posterior surface of the lens. When 

 the subject accommodates for near vision, the middle image alone 

 moves. It advances and grows smaller, showing that in the process 

 the anterior surface of the lens moves forward and becomes more 



Fio. 342. THE PHAKOSCOPE. 



convex. This phenomenon can also be demonstrated in daylight by 

 means of the instrument known as the phakoscope (Fig. 342). It 

 consists of a dark box, roughly triangular in shape, with the angles of 

 the triangle bevelled off, and at S and fitted with windows (Fig. 343). 



FIG. 343. DIAGRAM OF THE COURSE OF THE RAYS OF LIGHT IN THE PHAKOSCOPE. 



The observer's eye is at 0, the subject's at S. At G two prisms 

 are arranged vertically, and these are illuminated by a lamp, so as 

 to allow two illuminated squares to fall upon the eye placed at S. 

 The eye at S can either be focussed for the vertical needle placed at 

 the opening W, or for a distant object beyond the opening. With 



