Quite apart from the discovery of non-Euclidean geometry, the field of geometry made enormous strides in the nineteenth century. Much of this work, in both its synthetic and analytic forms, can be traced to the inspiring teaching of Gaspard Monge. Projective geometry, as an individual branch of mathematics, began in 1822 with the publication of the Traite des proprieties of Jean Victor Poncelet (1788-1867), perhaps Monge’s most outstanding student. This work of Poncelet gave a tremendous impetus to the study of the subject and inaugurated the “great period” in the history of projective geometry. As we have seen, in Section 9-8, the idea of poles and polars in projective geometry was elaborated by Poncelet and Joseph-Diez Gergonh (1771-1859) into a regular method out of which grew the elegant principle of duality. Many of Poncelet’s ideas were further dcveloped by the Swiss geometer Jacob Steiner (1796-1867), one of the greatest synthetic geometers the world has ever known. Projective geometry was finally completely freed of any metrical basis by Karl Georg Christian von Staudt (1798-1867) in has Geometrie der Lage of 1847. The analytical side of grometry made spectacular gains in the work of Augustus Ferdinand Mobius (1790-1868), Michel Chasles (1793-1880), and particularly, Julius Pluker (1801-1868). Michel chasles was also an outstanding synthetic geometer, and his Apercu historique sur l’origine cl le developpement das methods en geometrie (1837) is still a standard historical work. Felix Klein (1849-1929) introduced his Erlanger Programm for the codifition of geometries in 1872. It was shown how, by the adoption of a suitable projective definition of a metric, we can study metric geometry in the framework of projective geometry, and by the adjunction of an invariant to a projective geometry in the plane we can obtain the classical non-Euclidean geometries. In the late nineteenth and early twentieth centuries, projective geometry received a number of postulational treatments, and finite geometries were discovered. It was shown that, gradually adding and altering postulates, one can pass from projective geometry, encountering a number of other important geometries on the way. Finally, differential geometry, which alsa started in an essential way with Gaspard Monge, developed deeply in the nineteenth and twentieth centuries. This field of geometry received highly directive contributions from Carl Friedrich Gauss and Geory Bernhard Riemann. The last half of the nineteenth century saw the birth of the great Italian of geometers.