DISTANCE

(Lat. focus, a licarth.) In Optics, the distance between the centre of a lens er minor, and the point into which the rays are collected. FOCUS, in Geometry, is npplied to certain points belonging to conic sections, which po e very remarkable properties. The focus of the parabola is a point in the axis having this property, that a radius drawn from it to any point in the curve makes the same angle with the tangent at that point that the tangent makes with the axis. Hence a ray of light proceeding from the focus, and reflected by the curve, proceeds in a direction parallel to the axis; or, if parallel rays fall on the concave side of a parabola, they are reflected into the focus. In the ellipse the two foci are situated in the greater axis, at equal distances from the centre; and if from both foci straight lines be drawn to the same point ir. the circumference, the two lines make equal angles with the tangent at that point. A ray of light, therefore, i uing from the one focus, is reflected by the curve into the other focus. In the hyperbola a similar pro)>orty holds good; with this difference, that whereas in the ellipse the two straight lines fall both on the concave side of the arc. in the hyperbola ore falls on the concave and the other on the convex; in other words, the two lines drawn from the foci to any point [s. 476]