foci
 focus的複數形
 Fo·cus n.; pl. E. Focuses L. Foci
 1. Opt. A point in which the rays of light meet, after being reflected or refracted, and at which the image is formed; as, the focus of a lens or mirror.
 2. Geom. A point so related to a conic section and certain straight line called the directrix that the ratio of the distance between any point of the curve and the focus to the distance of the same point from the directrix is constant.
 Note: ☞ Thus, in the ellipse FGHKLM, A is the focus and CD the directrix, when the ratios FA:FE, GA:GD, MA:MC, etc., are all equal. So in the hyperbola, A is the focus and CD the directrix when the ratio HA:HK is constant for all points of the curve; and in the parabola, A is the focus and CD the directrix when the ratio BA:BC is constant. In the ellipse this ratio is less than unity, in the parabola equal to unity, and in the hyperbola greater than unity. The ellipse and hyperbola have each two foci, and two corresponding directrixes, and the parabola has one focus and one directrix.
     In the ellipse the sum of the two lines from any point of the curve to the two foci is constant; that is: AG + GB = AH + HB; and in the hyperbola the difference of the corresponding lines is constant. The diameter which passes through the foci of the ellipse is the major axis.  The diameter which being produced passes through the foci of the hyperbola is the transverse axis. The middle point of the major or the transverse axis is the center of the curve. Certain other curves, as the lemniscate and the Cartesian ovals, have points called foci, possessing properties similar to those of the foci of conic sections.
     In an ellipse, rays of light coming from one focus, and reflected from the curve, proceed in lines directed toward the other; in an hyperbola, in lines directed from the other; in a parabola, rays from the focus, after reflection at the curve, proceed in lines parallel to the axis. Thus rays from A in the ellipse are reflected to B; rays from A in the hyperbola are reflected toward L and M away from B.
 3. A central point; a point of concentration.
 Aplanatic focus. Opt. See under Aplanatic.
 Conjugate focus Opt., the focus for rays which have a sensible divergence, as from a near object; -- so called because the positions of the object and its image are interchangeable.
 Focus tube Phys., a vacuum tube for Rœntgen rays in which the cathode rays are focused upon the anticathode, for intensifying the effect.
 Principal focus, or Solar focus Opt., the focus for parallel rays.
 focus
      n 1: the concentration of attention or energy on something; "the
           focus of activity shifted to molecular biology"; "he had
           no direction in his life" [syn: focusing, focussing,
            direction, centering]
      2: maximum clarity or distinctness of an image rendered by an
         optical system; "in focus"; "out of focus"
      3: maximum clarity or distinctness of an idea; "the controversy
         brought clearly into focus an important difference of
         opinion"
      4: a central point or locus of an infection in an organism;
         "the focus of infection" [syn: focal point, nidus]
      5: special emphasis attached to something; "the stress was more
         on accuracy than on speed" [syn: stress]
      6: a point of convergence of light (or other radiation) or a
         point from which it diverges [syn: focal point]
      7: a fixed reference point on the concave side of a conic
         section
      v 1: direct one's attention on something; "Please focus on your
           studies and not on your hobbies" [syn: concentrate, center,
            centre, pore, rivet]
      2: cause to converge on or toward a central point; "Focus the
         light on this image" [ant: blur]
      3: bring into focus or alignment; to converge or cause to
         converge; of ideas or emotions [syn: concenter, concentre,
          focalize, focalise]
      4: become focussed or come into focus; "The light focused"
         [syn: focalize, focalise] [ant: blur]
      5: put (an image) into focus; "Please focus the image; we
         cannot enjoy the movie" [syn: focalize, focalise, sharpen]
         [ant: blur]
      [also: foci (pl)]
 foci
      See focus