Chapter 4 - Types of Geometrical Optical Illusions:
Illusions of Contour
Illusions of Contour. - The optical 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 unclosed one appears a little flatter and with a slightly larger radius than the one that is closed.
Fig. 11. - Two equal semicircles.
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.
Fig. 12. - Arcs of the same circle.
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 Muller-Lyer optical illusion illustrated in Fig. 56.
Fig. 13. - Three incomplete but equal squares.
Some of the optical illusions presented later will be seen to involve the influence of contour. Examples of these are Figs. 55 and Fig 60. In the former, the horizontal base line appears to sag; in the latter, the areas appear unequal, but they are equal.