Poncelet, Jean Victor (1788–1867)
Jean Poncelet was a French mathematician who substantially advanced projective geometry. With Brianchon, he proved Feuerbach's theorem on the nine-point circle in 1820–21, and also suggested the theorem proved by Steiner and now called the Poncelot-Steiner theorem that Euclidean constructions can be done with a straightedge alone. As a soldier in Napoleon's army, he was captured and imprisoned in Russia. While in prison from 1813–14, he organized and wrote down his discoveries, and the result was published as Traité des propriétés projectives des figures (1822). To serve as an introduction to this work, he also wrote Applications d'analyse et de géométrie (2 vols., 1862–64).