IT42 



VISION. 



very slight close to the eye, becomes more marked as it is removed, and 

 then decreases, till at a certain distance it is no longer visible. Kundt 1 

 determined the distance at which the illusion of Zollner's pattern disap- 

 peared for different observers, and varied the size of the figure and 

 number and angle of the cross lines. This illusion disappeared at a 

 smaller distance from the eye when the number of cross lines was 

 increased, and at a smaller distance with an angle of 20 than with 



FIG. 410. 



one of 32 or 45. There was no illusion with an angle of 70. The 

 results of different observers differed to some extent, but the variations 

 under the different conditions were the same for all. In Fig. 409, Kundt 

 found the illusion increased by distance. 



The illusions described have been referred to retinal factors, to the 

 influence of movement, and to purely psychological factors. The 



physiological factor to 

 which they have been 

 referred by Hering 2 and 

 Kundt is the curvature 

 of the retina. Their 

 mode of explanation may 

 be illustrated by the 

 simple illusion in which 

 an empty space appears 

 shorter than an equal 

 space partially filled with 

 intervening points. It is 

 supposed that our estima- 

 tion of length, when two 

 points of the retina are 

 stimulated, is a function 

 of the chord of the seg- 

 ment of the retina in- 

 cluded between the two 

 points. If this is the case, 

 our judgment of the dis- 

 tance (adeb) in Fig. 411 

 will depend on the sum of the component retinal chords (Ve 1 , ed', and 

 d'a'\ while that of the uninterrupted distance (be) will depend on Vc r , 

 and the interrupted distance will consequently be over-estimated. 



The wrong estimation of angles is explained by Hering 3 on the 



1 Ann. d. Phys. u. Chem., Leipzig, 1863, Bd. cxx. S. 118. 

 "Beitr. z. Physiol.," 1861, Heft 1, S. 65. 3 Ibid., S. 71. 



FIG. 411. Kundt. 



