Optical Illusions of Contour

The optical illusions of this type, or exhibiting this influence, are quite numerous.

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.

Fig. 11. - Two equal semicircles.

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. 12. - Arcs of the same circle

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 Muller-Lyer optical illusion illustrated in Fig. 56.

Some of the optical illusions presented later will be seen to involve the influence of contour.

Fig. 13. - Three incomplete but equal squares.

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.

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