ABSTRACT
Sometimes we experience an illusion without knowing it. For instance lawn sprinklers, radar aerials and windmills, all of which rotate, are sometimes reported either as oscillating or as rotating in the wrong direction. Unless we approach these objects we may never discover the error in our perception. Probably the best-known example of the apparent reversal of rotary motion in depth is the Ames window. Ames used a trapezium-shaped flat metal sheet, cut and painted to resemble a window at an angle. When rotated on a vertical axis this flat sheet looks like a rectangular window and appears to flap back and forth, provided it is viewed monocularly, or at a distance. The movement is misperceived in a systematic fashion, due to the misleading information provided by the perspective transformation of the object. Ames explained the illusion in terms of the observer's previous interactions with his environment. He pointed out that retinal images from square objects are almost always trapezoid, because most of the time we are not square on to the objects. As we interact with the world we learn that trapezoid images signal rectangular objects at an angle to us, unless there are depth cues indicating no slant. Thus a trapezium-shaped image from an object made to look like a window is interpreted as a rectangular window turned at an angle. According to Ames, the window is perceived as oscillating because the longer end of the trapezium is always seen as closer. This theory is inadequate, however, since it cannot account for the fact that many other shapes appear to change direction of rotation, even though they do not all oscillate in a regular fashion. They include trapezia without mullions and shadows, circles, ellipses, and more random shapes which do not have any misleading depth information (Power and Day, 1973).
