SPATIAL PERCEPTION. 



1141 



well marked in black type. Such type appears very much reduced in size 

 with a dilated pupil, and a difference of apparent size to the two eyes may 

 be due to inequality of the pupils. This kind of micropsia disappears 

 with an artificial small pupil, and does not occur for white objects on a dark 

 ground. 



One kind of micropsia which has attracted much attention is that which 

 occurs after instillation of atropin. This may be due to the dilated pupil and 

 irradiation, but also occurs when this factor is excluded. It is then probably 

 an instance of a physiological micropsia, which occurs when an object is 

 observed lying beyond the fixation point, and is associated with an appearance 

 of greater distance. Objects nearer than the fixation point, on the other hand, 

 appear enlarged, and with this may be connected the macropsia which occurs 

 after instillation of eserin. Donders l and others have supposed that the 

 micropsia of atropin is due to greater effort of accommodation and consequent 

 idea of greater nearness, and that with this idea of greater nearness is 

 associated one of smaller size. This explanation is open to the two objections 

 that the micropsia is usually accompanied by an appearance of greater distance, 

 and that it occurs with complete paralysis of accommodation, and Donders 

 himself gave up this explanation in 1886. 2 The micropsia which occurs 

 with commencing presbyopia seemed to be in favour of this explanation, but 

 this probably depends on indistinct vision. 



Geometrical illusions.— A number of illusions, usually classed 

 together under the above heading, are of considerable interest for 



a 



Fig. 408. 



the theory of the physiological basis of spatial localisation. A few 

 of the more important are shown. In Fig. 408 the unfilled distance db 

 appears shorter than he. In Fig. 409 the line db is really continuous 

 with cd, but appears as if continued to e. In 

 Fig. 410 the parallel lines appear to converge 

 at each end. 



Figs. 409 and 410 and many similar 

 illusions, including the well-known Zollner's 

 pattern, may all be referred to the erroneous 

 estimation of angles, namely, to the over- 

 estimation of acute angles and the under- 

 estimation of obtuse angles. 



The influence of various conditions on 

 these illusions has been determined. The 

 illusions are present with instantaneous il- 

 lumination, although, according to Helmholtz, 

 the effect is less marked than when move- 

 ment of the eye is allowed. They are present 

 in the after-image (Hering). Zollner found • 

 that the maximum apparent deviation of the a 

 parallel lines in his pattern occurred when 

 the cross lines were at an angle of 30°. The illusion is marked when 

 his figure is observed with the eye at right angles to the paper; it 

 becomes more marked when looked at obliquely from in front, and 

 disappears when observed obliquely from either end. The illusion is 



1 Nederl. Lancet, 1851, S. 607. 



2 Ber. Ophth. Gesellsch., Heidelberg, 1886, S. 82. 



Fig. 409. 



