An American text-book of physiology . 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 ~ ~ Fi<—To illustrate the over-estima- than its breadth. tion of vertical lines. Irradiation. — Many other circumstancesaffect the accuracy of the spatial perception of the retina. One of the mostimportant 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 THE SENSE OF VISION. 795 well shown by the ordina
An American text-book of physiology . 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 ~ ~ Fi<—To illustrate the over-estima- than its breadth. tion of vertical lines. Irradiation. — Many other circumstancesaffect the accuracy of the spatial perception of the retina. One of the mostimportant 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 THE SENSE OF VISION. 795 well shown by the ordinary incandescent electric lamp, the delicate filament ofwhich is scarcely visible when cold, but when intensely heated by the electriccurrent glows as a broad band of light. The phenomenon is known as irra-diation, and seems to depend chiefly upon the above-described imperfectionsin the dioptric apparatus of the eye, in consequence of which points of lightproduce 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 byblack and the black square surrounded by white (Figure 248), being of thesame size, would in an ideally perfect eye produce images of the same size onthe retina, but owing to the imperfections of the eye the images are not sharply
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Keywords: ., bookautho, bookcentury1800, bookdecade1890, booksubjectphysiology
