VIII 



OCULAE MOVEMENTS 



423 



the large and the small black disc is equidistant from each, but it 

 seems closer to the large than to the small disc. Contrast effect 

 also accounts for the lact that the 

 judgment of any direction or di- 

 mension is influenced by the vicinity 

 of another different direction or 

 dimension. For example, the upper 

 edge of Fig. 211 is a straight line, but 

 it appears to be bent, owing to the 

 proximity of the lower edge, which 

 is really an obtuse angle (Bourdon). 



Among the optical illusions of this group, that shown in 

 Fig. 212 (Muller-Lyer) is classical. The two halves of the line 



FIG. 210, Baldwin's optical illusion. 



| FIG. 211. Bourdon's optical illusion. 



are equal in length, but the oblique lines fall at obtuse angles on 

 the ends of the right half, and at acute angles at the end of the 



FIG. 212. Muller-Lyer's optical paradox. 



left half ; this produces the illusion that the first half is longer 

 than the second half. 



(e) The illusions recently discovered by Botti in Kiesow's 



FIG. 213. Botti's illusions, based on Muller-Lyer's figures. 



laboratory are associated with those of Miiller-Lyer. Line a a' of 

 Fig. 213 is the same length as line b I', but appears to be longer. 

 On the other hand, line c c' appears less than d d', although it 

 is equal to it. In the left-hand figure the upper, horizontal side is 



