Fig. 16.—An illusion of contrast.
A striking illusion of contrast is shown in [Fig. 17], where the central circles of the two figures are equal, although the one surrounded by the large circles appears much smaller than the other. Similarly, in [Fig. 18] the inner circles of b and c are equal but that of b appears the larger. The inner circle of a appears larger than the outer circle of b, despite their actual equality.
Fig. 17.—Equal circles which appear unequal due to contrast (Ebbinghaus’ figure).
Fig. 18.—Equal circles appearing unequal owing to contrasting concentric circles.