Notes on the Illusions - Java Puzzlers: Traps, Pitfalls, and Corner Cases - page 326

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Impossible Figures
An impossible figure is a two-dimensional perspective drawing of a figure that cannot exist in three
dimensions. The arrangement of cubes on page 122 is based on an impossible figure drawn by
Swedish artist Oscar Reutersvärd in 1934. This drawing is thought to be the first impossible figure
ever devised. Reutersvärd devoted his career to impossible figures.
The Penrose Triangle (pages 36 and 126) is closely related to Reutersvärd's triangle of cubes, but it
was created independently by physicist Roger Penrose in 1954. The Penrose Stairway (page 63)
was created in the mid 1950s by geneticist and psychiatrist Lionel Penrose, the father of Roger
Penrose. The Penrose stairway forms the basis of M. C. Escher's famous lithograph Ascending and
Descending (1960).
The Three-Stick Clevis (pages 59 and 200) is also known by many other names, such as Widgit,
Poiuyt , and Impossible Trident . Its origins are unknown, but it dates back at least to 1964, when D.
H. Schuster wrote an article about it in the American Journal of Psychology . The clevis also graced
the cover of the March 1965 issue of MAD Magazine , held aloft by a smiling Alfred E. Neuman.
The Ambihelical Hex Nut (pages 154 and 249) and the Impossible Ring (pages 109 and 164) are two
more impossible figures of unknown origin.
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