Figure 1.

Example of a transparent cube and the two most common interpretations of its arrangement in space. (A) Photograph of the apparatus used for these observations. A cubic frame with sides 5 cm in length was constructed from brass tubing painted black. The cube was cemented to a featureless white surface that could be rotated at variable speed by a small electric motor; a white cowl was attached to the base to provide a relatively featureless background. (B) Illustration of the two most common perceptions of a transparent cube oriented as in A. (Left) The actual (top-down) interpretation. (Right) The illusory (bottom-up) interpretation. The front and top, or the front and bottom, faces have been opacified to render the illustrations of the two interpretations unambiguous in two dimensions. Although the two perceptions illustrated here are invariably observed, a number of other interpretations can be briefly seen with prolonged observation. These include the perception of the transparent cube as a two-dimensional design and an interpretation in which the proximal superior and distal inferior vertices both appear to be coming toward the observer as the apices of overlapping pyramids.