Ames window

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The Ames trapezoid or Ames window is an image on, for example, a flat piece of cardboard that seems to be a rectangular window but is, in fact, a trapezoid. Both sides of the piece of cardboard have the same image. The cardboard is hung vertically from a wire so it can rotate around continuously, or is attached to a vertical mechanically rotating axis for continuous rotation. When the rotation of the window is observed, the window appears to rotate through less than 180 degrees, though the exact amount of travel that is perceived varies with the dimensions of the trapezoid. It seems that the rotation stops momentarily and reverses its direction. It is therefore not perceived to be rotating continuously in one section but instead is misperceived to be oscillating. This phenomenon was discovered by Adelbert Ames, Jr. in 1947.