796 



AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



dated for the distance at which the objects are viewed. The effect of irradi- 

 ation is most manifest when the dark portion of the field of vision over which 

 the irradiation takes place has a considerable breadth. Thus the circular white 

 spots in Figure 249, when viewed from a distance of three or four meters, 

 appear hexagonal, since the irradiation is most marked into the triangular dark 

 space between three adjacent circles. A familiar example of the effect of irra- 

 diation is afforded by the appearance of the new moon, whose sun-illuminated 

 crescent seems to be part of a much larger circle than the remainder of the 

 disk, which shines only by the light reflected upon it from the surface of the 

 earth. 



Subdivided Space. A space subdivided into smaller portions by inter- 

 mediate objects seems more extensive than a space of the same size not so sub- 

 divided. Thus the distance from A to E (Fig. 250) seems longer than that from 



D E 



FIG. 250. To illustrate the illusion of subdivided space. 



to Cj though both are of the same length, and for the same reason the square 

 D seems higher than it is broad, and the square E broader than it is high, the 

 illusion being more marked in the case of D than in the case of E, because, as 

 above explained, vertical distances are, as a rule, over-estimated. 



The explanation of this illusion seems to be that the eye in passing over a 

 subdivided line or area recognizes the number and size of the subdivisions, 

 and thus gets an impression of greater total size than when no subdivisions 

 are present. 



A good example of this phenomenon is afforded by the apparently increased 

 extent of a meadow when the grass growing oh it is cut and arranged in hay- 

 cocks. 1 



The relations of lines to each other gives rise to numerous illusions of 

 spatial perception, among the most striking of which are those afforded by the 

 so-called " Zollner's lines/' an example of which is given in Figure 251. Here 



1 It is interesting to note that a similar illusion has been observed when an interval of time 

 subdivided by audible signals is compared with an equal interval not so subdivided (Hall and 

 Jastrow : Mind, xi. 62). 



