| 释义 |
focal, a.|ˈfəʊkəl|
[ad. mod.L. focālis, f. focus: see -al1. Cf. F. focal.]
1. Of or pertaining to a focus; collected or situated at a focus. focal point = focus 2.
1713Derham Phys. Theol. iv. iii. 126 note, Whether the Convexity or Concavity of the Drum collects those Rays into a focal Point, or scatters them. 1794G. Adams Nat. & Exp. Philos. II. xv. 174 You may, by means of the focal rays from this glass, char or burn a piece of wood. 1808J. Webster Nat. Phil. 185 The rays..will..converge to the focal point. 1855H. Spencer Princ. Psychol. (1872) I. iii. viii. 357 Perfect vision implies a focal adjustment of the eyes. 1862G. P. Scrope Volcanos 264 The residuum of..lava..in what may be called its focal reservoir.
fig.1755Young Centaur v. Wks. 1757 IV. 239 Human thought, whose scattered rays must be collected, as it were, to a focal point, in order duly to warm our devotion. 1855Macaulay Hist. Eng. IV. 175 Titus, in order to be near the focal point of political intrigue and faction, had taken a house within the precinct of Whitehall.
2. Math. focal axis (in a conic): the axis which passes through the real foci. focal conic (i.e. ellipse or hyperbola): in the modular method of generating quadrics, a locus of the modular foci. focal curve: the locus of foci of a surface. focal distance, (a) of the parabola: the distance between the focus and the vertex; (b) of the ellipse and hyperbola: the distance between the foci and the centre. focal lines: in a quadric cone, the degenerate focal hyperbola. focal property: any property of a geometrical locus involving the intersections of the locus with the absolute. † focal tangent: a tangent from which the position of the foci of a central conic may be determined. umbilical focal conic, in the umbilical method of generating quadrics, the locus of the focus.
1706W. Jones Syn. Palmar. Matheseos 250 If c h meet the Focal Tangent in τ. 1807Hutton Course Math. II. 118 A F the focal distance. 1885C. Leudesdorf Cremona's Proj. Geom. 255 The point in which a directrix cuts the focal axis. 1886P. Frost Solid Geom. (ed. 3) 147 The fixed point is called an umbilical focus..and the locus of the focus the umbilical focal conic.
3. Optics. focal aperture, focal capacity: see quots. focal distance or focal length (of a lens or mirror): the distance between the centre and the focus. focal plane: the locus of the foci of different systems of parallel rays refracted through a lens. Also often attrib., as focal plane shutter, a roller-blind shutter with a wide slit that moves across the front of the plate or film. focal point: the intersection of a focal plane with the axis of the lens.
1693E. Halley in Phil. Trans. No. 205. 960 The focal distance sought. 1753Phil. Trans. XLVIII. 170 An object⁓lens whose focal length is a little less. 1879G. C. Harlan Eyesight vi. 81 A double concave glass of twelve inches focal distance. 1895R. S. Heath Geom. Optics (ed. 2) 82 An incident system of parallel rays will then converge to a point on the common focal plane. Ibid. 83 It is clear that F is the first focal point of the combined system. 1905Westm. Gaz. 21 Oct. 18/2 In photographing rapidly moving objects the focal-plane shutter does give unmistakable distortion. 1906Ibid. 8 Sept. 14/2 In a piece of apparatus so complex as the focal-plane reflecting camera. 1918Photo-Miniature XV. Mar. (Gloss.), Focal aperture (relative aperture), a term expressing the speed of a lens... Focal capacity, an item in the specification of a camera denoting the greatest focal length of lens which can be used. Ibid., Focal-plane shutter, an exposure shutter of the roller-blind or curtain type, placed as nearly in contact with plate or film as possible. 1921H. G. Ponting Gt. White South 58 To make focal-plane photographic exposures. 1950[see between-lens shutter s.v. between adv. 4].
fig.1847Emerson Repr. Men, Swedenborg Wks. (Bohn) I. 315 A colossal soul, he lies vast abroad on his times..requires a long focal distance to be seen. 1860Patmore Faithf. for Ever 214 Love requires the focal space Of recollection or of hope, Ere it can measure its own scope.
4. Path. Localized or centrally localized, as focal disease, focal hæmorrhage, etc.
1890in Gould Med. Dict.
Hence ˈfocally adv., at a focus.
1839De Quincey War Wks. 1863 IV. 280 The force of European opinion, focally converged upon the subject. |