Principal focus

Focus \Fo"cus\ (f[=o]"k[u^]s), n.; pl. E. {Focuses}
   (f[=o]"k[u^]s*[e^]z), L. {Foci} (f[=o]"s[imac]). [L. focus
   hearth, fireplace; perh. akin to E. bake. Cf. {Curfew},
   {Fuel}, {Fusil} the firearm.]
   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.
      [1913 Webster]

   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.
      [1913 Webster]

   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.
         [1913 Webster]

   3. A central point; a point of concentration.
      [1913 Webster]

   {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 R[oe]ntgen 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.
      [1913 Webster]