Fig. 9.—Three squares of equal dimensions which appear different in area and dimension.
In [Fig. 9] the three squares are equal in dimensions but the different characters of the divisions cause them to appear not only unequal, but no longer squares. In [Fig. 10] the distance between the outside edges of the three circles arranged horizontally appears greater than the empty space between the upper circle and the left-hand circle of the group.
Fig. 10.—The vertical distance between the upper circle and the left-hand
one of the group is equal to the overall length of the group of three circles.
Illusions of Contour.—The illusions of this type, or exhibiting this influence, are quite numerous. In [Fig. 11] there are two semicircles, one closed by a diameter, the other unclosed. The latter appears somewhat flatter and of slightly greater radius than the closed one. Similarly in [Fig. 12] the shorter portion of the interrupted circumference of a circle appears flatter and of greater radius of curvature than the greater portions. In [Fig. 13] the length of the middle space and of the open-sided squares are equal. In fact there are two uncompleted squares and an empty “square” between, the three of which are of equal dimensions. However the middle space appears slightly too high and narrow; the other two appear slightly too low and broad. These figures are related to the well-known Müller-Lyer illusion illustrated in [Fig. 56]. Some of the illusions presented later will be seen to involve the influence of contour. Examples of these are [Figs. 55] and [60]. In the former, the horizontal base line appears to sag; in the latter, the areas appear unequal, but they are equal.
| Fig. 11.—Two equal semicircles. | Fig. 12.—Arcs of the same circle. |