878 



THE SENSE OF SIGHT 



effort to focalize these lines in succession than if the eyes can sweep 

 straight across the open field. Quite similarly, if two equally large 

 squares are subdivided by an equal number of horizontal or vertical 

 lines, the one subdivided horizontally will appear to be larger from 

 below upward and the other from side to side (Fig. 476). If we adjust 

 two lines of equal length at right angles to one another, the vertical 



// 



V/ ^"^ 



// \ 



// \ 

 // ^\ 



// X 

 // X 



// 

 // 



/ 





A/ \\ 



W 

 \\ 

 \\ 

 \\ 

 \\ 



w 



Y \\ 



/ 



/ 



\ 



/ 

 // 



// 



/ 

 / 



\ 



\ 

 \ 



/ 



Fig. 478. — Zollner's Lines. 



one will seem to be the longer of the two (Fig. 477). This deception 

 has been explained by assuming that the contraction of the internal 

 or external oblique muscle, required to visualize the horizontal line, 

 is effected with a slighter expenditure of energy than that of the 

 oblique muscles, required to trace the vertical line. In the latter case, 

 the tendency of the superior rectus to divert the eye inward, must be 



Fig. 479. 



-Muller-Lyer Figures to Show Illusion in Space Perception. The Lines 



A AND B ARE OF THE SaME LeNGTH. 



counteracted by the contraction of the inferior oblique which turns 

 the eye outward. 



A very striking illusion may also be produced by placing convergent 

 and divergent oblique lines upon two parallel lines of the same length. 

 In the first case, the latter will appear to come together, and in the 

 second case, to separate more widely. A similar effect may be ob- 



