Focus tube
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]

The Collaborative International Dictionary of English. 2000.

### Look at other dictionaries:

• 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… …   The Collaborative International Dictionary of English

• tube — I (New American Roget s College Thesaurus) n. pipe, hose, conduit; tunnel, subway; slang, television. See passage. II (Roget s IV) n. 1. [A hollow cylinder] Syn. conduit, hose, test tube, tubing, tunnel, loom, subway; see also pipe 1 . 2. [A… …   English dictionary for students

• tube — Synonyms and related words: Amtrak, L, Photronic cell, adjutage, amplifier, anode, attenuator, audio frequency tube, baggage train, ballast regulator, ballast tube, barrel, base, beam switching tube, bole, branch, cable railroad, cable railway,… …   Moby Thesaurus

• Aplanatic 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,… …   The Collaborative International Dictionary of English

• Conjugate 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,… …   The Collaborative International Dictionary of English

• 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,… …   The Collaborative International Dictionary of English

• Solar 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,… …   The Collaborative International Dictionary of English

• Video camera tube — In older video cameras, before the mid to late 1980s, a video camera tube or pickup tube was used instead of a charge coupled device (CCD) for converting an optical image into an electrical signal. Several types were in use from the 1930s to the… …   Wikipedia

• X-ray tube — An X ray tube is a vacuum tube that produces X rays. They are used in X ray machines. X rays are part of the electromagnetic spectrum, an ionizing radiation with wavelengths shorter than ultraviolet light. X ray tubes evolved from experimental… …   Wikipedia

• Vacuum tube — This article is about the electronic device. For experiments in an evacuated pipe, see free fall. For the transport system, see pneumatic tube. Modern vacuum tubes, mostly miniature style In electronics, a vacuum tube, electron tube (in North… …   Wikipedia