Descartes in the Geometric defined and considered the remarkable curves called of ter him the ovals of Descartes, or simply Cartesians, which will be again referred to.
Other descriptive names for ovals and re-entrant branches cutting themselves may be used when required; thus, in the last-mentioned case a simple form is that of a figure of eight; such a form may break up into two ovals or into a doubly indented oval or hour-glass.
A form which presents itself is when two ovals, one inside the other, unite, so as to give rise to a crunode - in default of a better name this may be called, after the curve of that name, a limacon.
In the case of a biaxal plate perpendicular to the bisector of the acute angle between the optic axes, the curves of constant retardation are approximately Cassini's ovals, and the lines of like polarization are equilateral hyperbolae passing through the points corresponding to the optic axes.
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