THE SENSE OF VISION. 349 



Hence we judge the upward motion of the eye to be greater because to accom- 

 plish it we make a greater effort than is required 

 for a horizontal movement of equal extent. 



The position of the vertical line bisecting the 

 horizontal one (in Fig. 158) aids the illusion, as 

 may be seen by turning the page through 90°, so 

 as to bring the bisected line into a vertical posi- 

 tion, or by looking at the lines in Figure 159, in 

 which the illusion is much less marked than in 

 Figure 158. 



The tendency to over-estimate the length of 

 vertical lines is also illustrated by the error 

 commonly made in supposing the height of the 

 crown of an ordinarv silk hat to be greater 



. . , , . Fig. 159.— To illustrate the over-estima- 



than its breadth. tion of ver ticai lines. 



Irradiation. — Many other circumstances 

 affect the accuracy of the spatial perception of the retina. One of the most 

 important of these is the intensity of the illumination. All brilliantly illumi- 

 nated objects appear larger than feebly illuminated ones of the same size, as is 

 well shown by the ordinary incandescent electric lamp, the delicate filament of 

 which is scarcely visible when cold, but when intensely heated by the electric 

 current glows as a broad band of light. The phenomenon is known as " irra- 

 diation," and seems to depend chiefly upon the above-described imperfections 

 in the dioptric apparatus of the eye, in consequence of which points of light 

 produce small circles of dispersion on the retina and bright objects produce 



Fig. 160.— To illustrate the phenomenon of irradiation. 



images with imperfectly defined outlines. The white square surrounded by 

 black and the black square surrounded by white (Figure 160), being of the 

 same size, would in an ideally perfect eye produce images of the same size on 

 the retina, but owing to the imperfections of the eye the images are not sharply 

 defined, and the white surfaces consequently appear to encroach upon the darker 

 portions of the field of vision. Hence the white square looks larger than the 

 black one, the difference in the apparent size depending upon the intensity of 

 the illumination and upon the accuracy with which the eve can be accommo- 

 dated for the distance at which the objects arc 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 



