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Chapter 1 - Introduction:
Understanding Optical Illusions
There are many facts affecting vision regarding which no theory is necessary. They speak for themselves. There are many equally obvious facts which are not satisfactorily explained but the lack of explanation does not prevent their recognition. In fact, only the scientist needs to worry over systematic explanations and theoretical generalizations. He needs these in order to invade and to explore the other unknowns where he will add to his storehouse of knowledge. A long step toward understanding is made by becoming acquainted with certain physical, physiological, and psychological facts of light, color, and lighting. Furthermore, acquaintance with the visual process and with the structure of the eye aids materially. For this reason the next two chapters have been added even at the risk of discouraging some readers.
In a broad sense, any visual perception which does not harmonize with physical measurements may be termed an "optical illusion."
Therefore, this term could include physical optical illusions obtained by means of lenses, mirrors and prisms and also optical illusions such as the mirage.
It could also include the physiological illusions of light and color such as after images, irradiation, and contrast, and the psycho physiological illusions of space and the character of objects. In fact, the scope of the following chapters is arbitrarily extended to include all these aspects, but confines consideration only to "static" illusions.
In a more common sense, attention is usually restricted to the last group; that is, to the psycho-physiological illusions attending the perception of space and the character of objects although motion is often included. It should be obvious that no simple or even single theory can cover the vast range of optical illusions considered in the broad sense because there are so many different kinds of factors involved. For this reason explanations will be presented wherever feasible in connection with specific optical illusions. However, in closing this chapter it appears of interest to touch upon the more generally exploited theories of optical illusions of the type considered in the foregoing restricted sense. Hypotheses pertaining to optical illusions are generally lacking in agreement, but for the special case of what might be more safely termed "geometrical-optical illusions" two different theories, by Lipps and by Wundt respectively, are conspicuous. In fact, most theories are variants of these two systematic "explanations" of optical illusions (in the restricted sense).
Lipps proposed the principle of mechanical esthetic unity, according to which we unconsciously give to every space form a living personality and unconsciously consider certain mechanical forces acting. Our judgments are therefore modified by this anthropomorphic attitude. For example, we regard the circle as being the result of the action of tangential and radial forces in which the latter appear to triumph. According to Lipps' theory the circle has a centripetal character and these radial forces toward the center, which apparently have overcome the tangential forces during the process of creating the circle, lead to underestimation of its size as compared with a square of the same height and breadth. By drawing a circle and square side by side, with the diameter of the former equal to the length of a side of the latter, this optical illusion is readily demonstrated. Of course, the square has a greater area than the circle and it is difficult to determine the effect of this disparity in area. In Figure 60 where the areas of the circle and square are equal and consequently the height of the former is considerably greater than the latter, is of interest in this connection. By experimenting with a series of pairs consisting of a circle and a square, varying in dimensions from equal heights to equal areas, an idea of the “shrinking” character of the circle becomes quite apparent.
Wundt does not attribute the optical illusion to a deception or error of judgment but to direct perception. According to his explanation, the laws of retinal image (fixation) and eye movement are responsible. For example, vertical distances appear greater than horizontal ones because the effort or expenditure of energy is greater in raising the eyes than in turning them through an equal angle in a horizontal plane. Unconscious or involuntary eye-movements also appear to play a part in many linear or more accurately, angular illusions, but certainly Wundt's explanation does not suffice for all optical illusions although it may explain many geometrical illusions. It may be said to be of the "perceptive" class and Lipps' theory to be of the "judgment" or "higher-process" class. As already stated, most of the other proposed explanations of geometrical illusions may be regarded as being related to one of these two theories. There is the "indistinct vision" theory of Einthoven; the "perspective" theory of Hering, Guye, Thiery, and others; the "contrast" theory of Helmholtz, Loeb, and Heyman; and the "contrast-confluxion" theory of Muller-Lyer. In order not to discourage the reader at the outset, theories as such will be passed by with this brief glimpse. However, more or less qualified explanations are presented occasionally in some of the chapters which follow in order to indicate or to suggest a train of thought should the reader desire to attempt to understand some of the numerous interesting optical illusions.
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