focus (fo'kus)

PI. foci. 1. The point at which rays (especially of light) are collected or converged by a lens or mirror. When used without qualification, the principal f. (q. v.) is always meant. 2. A central point, or point of departure; a nucleus; a point at which a proce is manifested in great intensity, acoustic f. The point of meeting of sound waves reflected from a concave surface, anterior f. That point in front of a system of refracting surfaces at which parallel rays impinging upon this system from behind are united; the anterior principal f. of a compound dioptric system, anterior f. of the eye. That point in front of the eye at which rays coming from the eye and parallel to the principal axis are united. Light placed at this point sends rays which penetrate the eye 353 as parallel rays, calorific f. The point at which rays of heat (the red and ultrared rays of the spectrum) are collected by a lens or mirror, conjugate foci. Two points of such a position that light rays emanating from one are collected by a mirror or lens at the other. Conjugate foci are mutually convertible, i. e., either one may be taken as the point of emanation of the rays or vibrations, and also as the point of collection of the latter. depth of f. The distance through which an image formed at the f. of a lens may be shifted without materially impairing the definition of the image, first f. See anterior f. f. of a •lens, f. of a mirror. See principal f. f. of the eye. See anterior f. of the eye and principal f. of the eye. imaginary f., negative f. See virtual f. posterior f. That point posterior to a system of refracting surfaces at which rays impinging in a parallel direction upon the anterior surface of the system are united, posterior f. of the eye. See principal f. of the eye. posterior principal f. See posterior f. primary f., principal f. The point at which parallel rays falling on a lens or mirror are collected. The principal f. of a concave lens or of a convex mirror is virtual; of a convex lens or concave mirror, real. According to the laws of conjugate foci, rays emanating from the principal f. are reflected or refracted in parallel directions, principal f. of the eye, principal posterior f. That point posterior to the refractive media of the eye at which rays impinging upon the cornea in a direction parallel to the optic axis are united. In emmetropic eyes the principal f. is upon the retina; in myopic eyes, in front of it; in hypermetropic eyes, behind it. real f. A point in front of a mirror or behind a lens at which rays made convergent by reflection or refraction meet, sonorous f. Point at which vibrations of sound are collected by a mirror or by a lens composed of such material as to refract them, thermal f. See calorific f. virtual f. A point behind a mirror or in front of a lens at which rays made divergent by reflection or refraction would meet if prolonged. [Lat., focus, hearth.]