What is "cartesion oval"

Cartesian \Car*te"sian\, a. [From Renatus Cartesius, Latinized from of Ren['e] Descartes: cf. F. cart['e]sien.] Of or pertaining to the French philosopher Ren['e] Descartes, or his philosophy.

The Cartesion argument for reality of matter.
--Sir W. Hamilton.

Cartesian co["o]rdinates (Geom), distance of a point from lines or planes; -- used in a system of representing geometric quantities, invented by Descartes.

Cartesian devil, a small hollow glass figure, used in connection with a jar of water having an elastic top, to illustrate the effect of the compression or expansion of air in changing the specific gravity of bodies.

Cartesion oval (Geom.), a curve such that, for any point of the curve mr + m'r' = c, where r and r' are the distances of the point from the two foci and m, m' and c are constant; -- used by Descartes.