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Moldering in a Russian prison, Poncelet founded a new discipline: projective geometry.

But Gergonne would surely have approved: he studied projective geometry, which involves shapes with exotic topologies, such as surfaces that have only one side.

The others are said to be projective properties, and their investigation forms the subject of projective geometry.

And this is true even of such apparently essentially intuitional fields as projective geometry, where entities can be substituted for directional lines and the axioms be turned into relational postulates governing their configurations.

The author is professor of projective geometry and geometric drawing in the University of Bologna, and is one of the most conspicuous of contemporary Italian scientists.