"The 19th-century French magician Jean-Eugène Robert-Houdin, one of the greatest illusionists of his time, also applied his love of optical trickery to his first profession: clockmaking.
Among the mechanical illusions that he is credited with inventing, one stands out: the mystery movement, so called because the hands of the clock appeared to be floating over a transparent dial."
Article at The New York Times>>
For more on mystery clocks, see this page by Jim Linz>>
And this article at Cigar Aficionado>>
More mystery clock goodness by Roger Russell at the Mystery Clock History Page>>
(Pictured: Haddon mystery clock, circa 1950, thanks to Daryls Clocks Galore)