THE SENSE OF VISION. 795 



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. 248. 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 248), 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 



FIG. 249. To illustrate the phenomenon of irradiation. 



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 eye can be accommo- 



