592 



PHYSIOLOGY 



MMfSMffffffL 

 iXXXXXXXXXXXXXXXXX 



movements is due the illusion in Fig. 304, where parallel lines seem to 

 diverge or converge. Of two equal lines, one of which is vertical and one 

 horizontal, the vertical line seems the longer. When the eye is 



moved upwards, the tendency of the 

 superior rectus to move the eye inwards 

 has to be counteracted by a pull of the 

 inferior oblique muscle turning the eye 

 outwards. A greater effort is therefore 

 required than when the eye is moved 

 either inwards or outwards, and it has 

 been suggested that it is this greater 

 muscular effort which is responsible 

 for our over- valuation of the length of 

 |^j^X\2^^X^^^j^^^^^y vertical as compared with that of hori- 

 ^x^vvvvvxxxxxxxxx zontal lines. 



(j It must not, however, be imagined 



FIG 304 that an actual movement of the eyes 



is necessary in order to judge of the 



size and distance of any object. If a white thread be hung up in a dark 

 room and be illuminated for an instant of time by an electric spark it is 

 impossible for an observer in the dark room to move his eyes so that the 



L.E. 



FIG. 305. The eyes are directed to the point b. A thread hung obliquely at a 

 under these circumstances gives rise to the images shown in the upper figures 

 i.e. two images which do not lie on corresponding points. Nevertheless the 

 thread is seen as single. 



image of the thread shall fall on the corresponding points of the two retinae 

 before the illumination has disappeared. In spite of the fact, however, 

 that the image of the thread falls on non-corresponding points, as will 

 be seen from the diagram (Fig. 305), the thread is seen, not as double, 



