Axis

Axis, in Geometry. The axis of a curved line is formed by a right line dividing the curve into two symmetrical parts, so that the part on one side exactly corresponds with that on the other; as in the parabola, the ellipse, and the hyperbola. The axis of any geometrical solid is the right line which passes through the centre of all the corresponding parallel sections of it: in this sense, we speak of the axis of a cylinder, a globe, or a spheroid. By the axis of rotation, we understand the right line around which a body revolves.—In physical science, the axis of a lens is the right line passing through it in such a manner as to be perpendicular to both sides of it; and the axis of a telescope is a right line which passes through the centres of all the glasses in the tube. The axis of the eye is the right line passing through the centres of the pupil and the crystalline lens.