VIII 



OCULAR MOVEMENTS 



421 



contrary, wider than it is high. Square c, again, like b appears 

 higher than its width, but a seems higher than c ; d, again, divided 

 by a lattice-work, looks larger than the empty square c. 



(&) The straight line a, divided in the middle, is the same 

 length as the straight line a', but appears shorter (Fig. 206). If, 

 however, the mark is moved nearer the end of the line, as in &, it 

 gradually begins to look longer than V (Botti). Again, if a dot 



y 



Fin. 206. Optical illusions. (Botti (a, a', 1>, ?/) and Kiesow (c, c').) 



is placed at a certain distance above the centre of the straight 

 line, as in c, and then moved to the right or left, as in c', the line 

 seems to lengthen gradually (Kiesow). 



(c) Acute angles are generally judged as greater, obtuse angles 

 as less than they really are (Helmholtz). This error underlies a 

 whole series of optical illusions, the best known of which are 

 illustrated by the diagrams of Hering and Zollner. In Fig. 207, 



at 



FIG. 207. Bering's lines : optical illusion. 



the four horizontal lines are perfectly straight and parallel, yet 

 the upper pair seem to approximate in the middle and to diverge 

 at their ends, while the lower pair, in which oblique lines 

 drawn in the opposite directions, appear to diverge at their 

 centres. In Fig. 208, the six black horizontal lines are absolutely 

 parallel to each other, but each pair of lines appears to converge 

 from left to right, owing to the presence of oblique lines which 

 run in the opposite direction. The parallelism of the first and 



