An Isoptic Curve formed from the locus of Tangents meeting at Right Angles. The orthoptic of a Parabola is its Directrix. The orthoptic of a central Conic was investigated by Monge and is a Circle concentric with the Conic Section. The orthoptic of an Astroid is a Circle.

Curve | Orthoptic |

Astroid | Quadrifolium |

Cardioid | Circle or Limaçon |

Deltoid | Circle |

Logarithmic Spiral | equal Logarithmic Spiral |

Parabola | Directrix |

**References**

Lawrence, J. D. *A Catalog of Special Plane Curves.* New York: Dover, pp. 58 and 207, 1972.

© 1996-9

1999-05-26