Fig. 13.—Three incomplete but equal squares.
Illusions of Contrast.—Those illusions due to brightness contrast are not included in this group, for “contrast” here refers to lines, angles and areas of different sizes. In general, parts adjacent to large extents appear smaller and those adjacent to small extents appear larger. A simple case is shown in [Fig. 14], where the middle sections of the two lines are equal, but that of the shorter line appears longer than that of the longer line. In [Fig. 15] the two parts of the connecting line are equal, but they do not appear so. This illusion is not as positive as the preceding one and, in fact, the position of the short vertical dividing line may appear to fluctuate considerably.
Fig. 14.—Middle sections of the two lines are equal.
Fig. 15.—An effect of contrasting areas (Baldwin’s figure).
[Fig. 16] might be considered to be an illusion of contour, but the length of the top horizontal line of the lower figure being apparently less than that of the top line of the upper figure is due largely to contrasting the two figures. Incidentally, it is difficult to believe that the maximum horizontal width of the lower figure is as great as the maximum height of the figure. At this point it is of interest to refer to other contrast illusions such as [Figs. 20], [57], and [59].