资料来源 : Webster's Revised Unabridged Dictionary (1913)

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.

            Heat and cold have a virtual transition, without
            communication of substance.           --Bacon.

            Every kind that lives, Fomented by his virtual
            power, and warmed.                    --Milton.

   2. Being in essence or effect, not in fact; as, the virtual
      presence of a man in his agent or substitute.

            A thing has a virtual existence when it has all the
            conditions necessary to its actual existence.
                                                  --Fleming.

            To mask by slight differences in the manners a
            virtual identity in the substance.    --De Quincey.

   {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
      work}.

   {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.

Image \Im"age\, n. [F., fr. L. imago, imaginis, from the root of
   imitari to imitate. See {Imitate}, and cf. {Imagine}.]
   1. An imitation, representation, or similitude of any person,
      thing, or act, sculptured, drawn, painted, or otherwise
      made perceptible to the sight; a visible presentation; a
      copy; a likeness; an effigy; a picture; a semblance.

            Even like a stony image, cold and numb. --Shak.

            Whose is this image and superscription? --Matt.
                                                  xxii. 20.

            This play is the image of a murder done in Vienna.
                                                  --Shak.

            And God created man in his own image. --Gen. i. 27.

   2. Hence: The likeness of anything to which worship is paid;
      an idol. --Chaucer.

            Thou shalt not make unto thee any graven image, . .
            . thou shalt not bow down thyself to them. --Ex. xx.
                                                  4, 5.

   3. Show; appearance; cast.

            The face of things a frightful image bears.
                                                  --Dryden.

   4. A representation of anything to the mind; a picture drawn
      by the fancy; a conception; an idea.

            Can we conceive Image of aught delightful, soft, or
            great?                                --Prior.

   5. (Rhet.) A picture, example, or illustration, often taken
      from sensible objects, and used to illustrate a subject;
      usually, an extended metaphor. --Brande & C.

   6. (Opt.) The figure or picture of any object formed at the
      focus of a lens or mirror, by rays of light from the
      several points of the object symmetrically refracted or
      reflected to corresponding points in such focus; this may
      be received on a screen, a photographic plate, or the
      retina of the eye, and viewed directly by the eye, or with
      an eyeglass, as in the telescope and microscope; the
      likeness of an object formed by reflection; as, to see
      one's image in a mirror.

   {Electrical image}. See under {Electrical}.

   {Image breaker}, one who destroys images; an iconoclast.

   {Image graver}, {Image maker}, a sculptor.

   {Image worship}, the worship of images as symbols; iconolatry
      distinguished from idolatry; the worship of images
      themselves.

   {Image Purkinje} (Physics), the image of the retinal blood
      vessels projected in, not merely on, that membrane.

   {Virtual image} (Optics), a point or system of points, on one
      side of a mirror or lens, which, if it existed, would emit
      the system of rays which actually exists on the other side
      of the mirror or lens. --Clerk Maxwell.

资料来源 : WordNet®

virtual image
     n : a reflected optical image (as seen in a plane mirror)