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Definition: Virtual focus
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;
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.
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.