At the end of the 19th century and during the first quarter of the 20th century, artists, musicians, novelists, and poets were fascinated by non-Euclidean geometry and the fourth dimension and how to portray these concepts. Their interest in mathematics, as well as in recent scientific discoveries such as electromagnetic waves (confirmed in 1888), X-rays (1895), and electrons (1897), provoked artists to look at the world in a new way and to record their observations in paintings and sculpture. Robert Delaunay (1885-1941) rejoiced in the visual impact of colors, as is apparent in his famous 1931 painting Rhythme: Joie de Vivre reproduced on this stamp. The circles, broken and unbroken, represent the halos around glowing electric street lamps. Interpreting this work, Delaunay remarked "Tout est halo," adding that he had never seen a straight line in his life. Earlier he painted many versions of the Eiffel tower, celebrated by Parisians as the location for the transmission of Hertzian waves that made long-range communication possible. In addition to this interest in radio waves, Delaunay explored the wave nature of light and color. Delaunay and his contemporaries were aware of the new geometries, the work of Victor Schiegel, and the writings of Jules Henri Poincar6, in part through the actuary Maurice Princet, who was deeply involved with Parisian artists at the beginning of the twentieth century. In 1912, Princet wrote the catalog for Delaunay's first exhibition, which was held that year. Broadway Boogie-Woogie of 1943 is one of the last three paintings of jazzloving Piet Mondr ian (1872-1944), in which he creates spatial ambiguity by colored (no longer black) lines. For years, Mondrian shared the enthusiasms of other artists in portraying the fourth dimension. He wrote that in his work, the negative or white space represented the fourth dimension. Mondrian attempted to portray the universal mathematical harmonies behind visual phenomena. Eventually, he rejected 3-dimensional visual reality as a superficial illusion. He believed that two dimensions represented objects clearly but that perspective representation, implying three dimensions, weakens them. Consequently, he made sure that all overlapping shapes and diagonals, which could be construed as 3-dimensional, were eliminated from his work. Optical art stimulates and confuses our perception. In Vega-chess, Victor Vasarely (1908-97) utilised this illusion by repeating the same motif over and over, making it difficult for us to see the picture. The painting, which on close scrutiny appears to have no background, can be hung with any side at the top because the picture has the eight symmetries of a square. His Tr/mensional design is similarly based on symmetry. Vasarely said that he was influenced by isochromatic lines in physics and by the isobar maps he studied in school geography.