THE SENSE OF VISION. 797 



the horizontal lines, though strictly parallel to each other, seem to diverge and 

 converge alternately, their apparent direction being changed toward greater per- 



x x x x XXXX X X X 



xVVV/VVWW 





X \ XXXXXXXXX 



\\ \\X\\\\ N\ 



FIG. 251. Zollner's lines. 



pendicularity to the short oblique lines crossing them. This illusion is to be ex- 



plained in part by the tendency of the eye to over-estimate the size of acute and to 



uuder-estimate that of obtuse angles a tendency which 



also affords a partial explanation of the illusion in 



Figure 252, where the line d is the real and the line/ 



the apparent continuation of the line a. The illusion 



in Zollner's figures is more marked when the figure is 



so held that the long parallel lines make an angle of 



about 45 with the horizon, since in this position the 



eye appreciates their real position less accurately than 



when they are vertical or horizontal. It is dimin- 



ished, but does not disappear, when the eye, instead 



of being allowed to wander over the figure, is fixed 



upon any one point of the field of vision. Hence the 



, , , . FIG. 252. To illustrate illusion 



motions of the eye must be regarded as a factor m, but of space-perception. 

 not the sole cause of, the illusion. 



Our estimate of the size of given lines, angles, and areas is influenced by 

 neighboring lines, angles, and areas with which they are compared. This 

 influence is sometimes exerted in accordance with the principle of contrast, 

 and tends to make a given extension appear larger in presence of a smaller, 



FIG. 253. To illustrate contrast in space-perception (Muller-Lyer). 



and smaller in presence of a larger extension. This effect is illustrated in 

 Figure 253, in which the middle portion of the shorter line appears larger 

 than the corresponding portion of the longer line, in Figure 254, in which a 

 similar effect is observed in the case of angles, and in Figure 255, in which 



