VISION, 587 
up or down, to the right or the left, he may ascertain the limits 
of the visual field for a plane surface. The visual field for 
both eyes measures about 180° in the horizontal meridian; for 
one eye about 145°; and in the vertical meridian 100°. 
Imperfections of Visual Perceptions—We may now consider 
some defects which we know to exist by the use of our reason- 
ing powers in the mental perception we form of objects in the 
visual field : 
1. Irradiation.—It is easy to notice that a white spot ona 
dark ground appears larger than a dark spot of equal size on 
a white ground. This has been spoken of as the result of 
Fia. 428.—Illustrates irradiation. The white patch in the dark ground seems larger than the 
dark one in the light ground (after Bernstein). 
irradiation—a sort of overflow of sensation, though whether 
to be referred to the retina or to the brain-areas concerned is 
uncertain. 
2. Contrast—When a white strip of paper is laid between 
two black ones, the center of the white strip is not so bright as 
its edges, from contrast; and experiments illustrating the same 
principle may be made with colored paper. This law of con- 
trast is very wide in its application, and will be referred to 
later. 
3. The Blind-Spot.—tIt might be supposed at first that one 
should perceive gaps in the field of vision on account of the 
blind-spot; but, when it is remembered that to see black we 
must have a definite sensation, and that the mind places objects 
lying on opposite sides of the spot close together, the reason 
that this defect in structure, if such it really be, is practically 
inoperative, becomes clearer. It is to be remembered that the 
image of an object (see Fig. 432) never falls on the blind-spot 
in both eyes; and, moreover, this area lies outside of that 
of greatest acuteness (macula lutea), on which images are 
focused. 
The macula lutea, and especially the fovea centralis, are the 
