# Cone

## DEFINITION

1. A three-dimensional shape having a surface formed by a straight line (the side length) passing through a fixed point and moving along a circular curve. It usually refers to a right circular cone ? a cone in which the side length remains a constant length. The volume of a right circular cone equals one-third of pi (3.14159) times radius squared times height. Its surface area (including circular side) equals pi times radius times the sum of radius plus side length. Lateral area (not including circular side) equals pi times radius times side length.Related link: A diagram of conic sections including a circle, an ellipse, a parabola, and a hyperbola. Also see arc, circle, the second sense of "cone", coning, cylinder, ellipse, hyperbola, parabola, polyhedron, pyrometric cones, and sphere.2. Physically essential to enabling sight, the photoreceptors on the retina of the eye which are responsive to color and in bright conditions. see thumbnail to rightIn this illustration, the cones and rods are the pink forms shown embedded in the anterior of the retina's surface. The cones are the thicker, conical forms, among the more uniformly thin rods.Also see afterimage, binocular vision, colorblind, the first sense of "cone", gestalt, Op Art, ophthalmology, optical, optical mixing, perception, and peripheral vision.