Gérard Desargues (zhārär´ dəzärg´), 1591–1661, French mathematician and engineer, a founder of modern geometry. He discovered the theorems on involutions and transversals known by his name and worked on conic sections. His writings, lost for a time, were republished in 1864. His purely mathematical texts, in French, ed. by René Taton, were republished in 1951.