422 



PHYSIOLOGY 



CHAP. 



third lines is obvious, because the oblique lines which cross 

 them lie in the same direction. Interesting variants of this 

 group of illusions were discovered by Preobrajenski. Fig. 209, A, 

 shows a circle in the centre, but it appears to be flattened 



FIG. 208. Zollner's lines : optical illusion. 



towards the left, where it is intersected by an acute angle, and 

 more curved towards the right, where it is crossed by lines at an 

 obtuse angle. Fig. 209, B, shows a square with double outlines, 

 but the superior angle on the right appears acute, because it is 



FIG. 200. Preobrajenski's optical illusion. 



intersected obliquely by lines which meet each other at an 

 obtuse angle. 



(d) Some optical illusions seem due to contrast effects (Miiller- 

 Lyer). Two equal lines between two objects of 'different sizes 

 appear unequal in length. The plainest illustration of this illu- 

 sion is given by Baldwin (Fig. 210). The dot interposed between 



