Chapter 4 - Types of Geometrical Optical Illusions:
Illusions of Contrast

Illusions of Contrast. - The optical illusions that are due to brightness contrast will not be included in the Optical illusions of Contrast group because "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.

Fig. 14. - Middle sections of the two lines are equal.

In Fig. 15 the two parts of the connecting line are equal, but they do not appear so. This optical 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. 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 Fig 20, Fig 57, and Fig 59.

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, though the one surrounded by the large circles appears much smaller than the other.

Fig. 17. - Equal circles which appear unequal due to contrast (Ebbinghaus' figure).

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. 18. - Equal circles appearing unequal owing to contrasting concentric circles.

In Fig. 19 the circle nearer the apex of the angle appears larger than the other. This has been presented as one reason why the sun and moon appear larger at the horizon than when at higher altitudes. This explanation must be based upon the assumption that we interpret the "vault" of the sky to meet at the horizon in a manner somewhat similar to the angle but it is difficult to imagine such an angle made by the vault of the sky and the earth's horizon. If there were one in reality, it would not be seen in profile.

Fig. 19. - Circles influenced by position within an angle.

If two angles of equal size are bounded by small and large angles respectively, the apex in each case being common to the inner and two bounding angles, the effect of contrast is very apparent, as seen in Fig. 20. In Fig. 57 are examples of effects of lines contrasted as to length.

Fig. 20. - Contrasting angles.

The reader may readily construct an extensive variety of illusions of contrast; in fact, contrast plays a part in most geometrical-optical illusions. The contrasts may be between existing lines, areas, etc., or the imagination may supply some of them.

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