Focal

a. [ad. mod.L. focālis, f. FOCUS: see -AL. Cf. F. focal.]

  1.  Of or pertaining to a focus; collected or situated at a focus. Focal point = FOCUS 2.

1713.  Derham, Phys. Theol., IV. iii. 126, note. Whether the Convexity or Concavity of the Drum collects those Rays into a focal Point, or scatters them, &c?

1794.  G. 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 to a coal in a decanter of water.

1808.  J. Webster, Nat. Phil., 185. When the radiant, or object, is at a considerable distance from the lens, the rays issuing from it will fall upon the glass and converge to the focal point; whence the image will appear inverted, but clear and distinct, as if the object was placed there in the same position.

1855.  H. Spencer, Princ. Psychol. (1872), I. III. viii. 357. Perfect vision implies a focal adjustment of the eyes.

1862.  G. P. Scrope, Volcanos, 263–4. The residuum of molten or liquefied lava in the vent, or in what may be called its focal reservoir beneath, may be reconsolidated.

  fig.  1753.  Young, Centaur, v. Wks. 1757, IV. 239. What is this world, but a machine played on by us by our great enemy for the dissipation of human thought, whose scattered rays must be collected, as it were, to a focal point, in order duly to warm our devotion; and set a pious heart on fire?

1855.  Macaulay, 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 center. 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.

1706.  W. Jones, Synopsis Palmariorum Matheseos, 250. If cb meet the Focal Tangent in τ.

1807.  Hutton, Course Math., II. 118. AF the focal distance.

1885.  Leudesdorf, Cremona’s Proj. Geom., 255. The point in which a directrix cuts the focal axis.

1886.  P. 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 distance or length (of a lens or mirror): the distance between the center and the focus. Focal plane: the locus of the foci of different systems of parallel rays refracted through a lens. Focal point: the intersection of a focal plane with the axis of the lens.

1693.  E. Halley, in Phil. Trans., No. 205. 960. The focal distance sought.

1753.  Phil Trans., XLVIII. 170. An object-lens whose focal length is a little less.

1879.  Harlan, Eyesight, vi. 81. A double concave glass of twelve inches focal distance.

1895.  R. 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.

  fig.  1847.  Emerson, Repr. Men, Swedenborg, Wks. (Bohn), I. 315. A colossal soul, he lies vast abroad on his times, uncomprehended by them, requires a long focal distance to be seen.

1860.  Patmore, Faithf. for Ever, 214.

  4.  Path. Localized or centrally localized, as focal disease, hæmorrhage, etc.

1890.  in Gould, Med. Dict.

  Hence Focally adv., at a focus.

1839.  De Quincey, War, Wks. 1863, IV. 280. When all the causes of war involving manifest injustice are banished by the force of European opinion, focally converged upon the subject, the range of war will be prodigiously circumscribed.