virtual focus

From The Collaborative International Dictionary of English v.0.48:

Virtual \Vir"tu*al\ (?; 135), a. [Cf. F. virtuel. See Virtue.]
   1. Having the power of acting or of invisible efficacy
      without the agency of the material or sensible part;
      potential; energizing.
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            Heat and cold have a virtual transition, without
            communication of substance.           --Bacon.
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            Every kind that lives,
            Fomented by his virtual power, and warmed. --Milton.
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   2. Being in essence or effect, not in fact; as, the virtual
      presence of a man in his agent or substitute.
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            A thing has a virtual existence when it has all the
            conditions necessary to its actual existence.
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            To mask by slight differences in the manners a
            virtual identity in the substance.    --De Quincey.
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   Principle of virtual velocities (Mech.), the law that when
      several forces are in equilibrium, the algebraic sum of
      their virtual moments is equal to zero.

   Virtual focus (Opt.), the point from which rays, having
      been rendered divergent by reflection of refraction,
      appear to issue; the point at which converging rays would
      meet if not reflected or refracted before they reach it. 

   Virtual image. (Optics) See under Image.

   Virtual moment (of a force) (Mech.), the product of the
      intensity of the force multiplied by the virtual velocity
      of its point of application; -- sometimes called {virtual

   Virtual velocity (Mech.), a minute hypothetical
      displacement, assumed in analysis to facilitate the
      investigation of statical problems. With respect to any
      given force of a number of forces holding a material
      system in equilibrium, it is the projection, upon the
      direction of the force, of a line joining its point of
      application with a new position of that point indefinitely
      near to the first, to which the point is conceived to have
      been moved, without disturbing the equilibrium of the
      system, or the connections of its parts with each other.
      Strictly speaking, it is not a velocity but a length.

   Virtual work. (Mech.) See Virtual moment, above.
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