[a. F. concave (14th c. Oresme), ad. L. concavus, f. con- + cavus hollow: see CAVE.]

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  † 1.  Having an internal hole or cavity; hollow.

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1571.  Digges, Pantom., I. xxx. The concaue Cylinders.

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1600.  Shaks., A. Y. L., III. iv. 26. I doe thinke him as concaue as a couered goblet, or a Worme-eaten nut.

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1659.  Willsford, Archit., 24. A concave vessel containing a cubical yard.

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  2.  Having the outline or surface curved like the interior of a circle or sphere; having a curvature that presents a hollow to the point of observation; the reverse of convex; incurvated.

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1594.  Blundevil, Exerc., III. I. (ed. 7), 271. The upper part of such a Vault is sayd to be Convex and the inward part Concave.

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1656.  Hobbes’ Elem. Philos. (1839), 279. If two strait converging lines … fall upon the concave circumference of a circle.

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1753.  Hogarth, Anal. Beauty, xii. 101. It will … appear concave like a bason.

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1816.  J. Smith, Panorama Sc. & Art, I. 39. The screw cut by a tap is called an inside or concave screw.

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1822.  Imison, Sc. & Art, I. 72. The grinding surface of the under [mill-]stone is a little convex … and that of the upper stone a little concave.

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1880.  Günther, Fishes, 51. The vertebra … with a concave anterior and posterior surface.

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  b.  esp. used of glasses, lenses, mirrors, etc., made in this form for optical purposes.

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1571.  Digges, Pantom., I. vi. Playne, conuex, or concaue glasses.

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1662.  Hobbes, 7 Philos. Probl., Wks. 1845, VII. 30. How comes the light of the sun to burn almost any combustible matter by refraction through a convex glass, and by reflection from a concave?

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1833.  Brewster, Nat. Magic, iv. 61. The concave mirror is the staple instrument of the magician’s cabinet.

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1869.  Tyndall, Notes on Light, § 156. Double concave, with both surfaces concave. Plano-concave, with one surface plane and the other concave.

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1878.  T. Bryant, Pract. Surg., I. 301. Until we have found the weakest concave lens with which distant letters can be most plainly seen.

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  3.  Comb., as concave-planned adj.

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1879.  Sir G. Scott, Lect. Archit., I. 84. A rudimental relic of the concave-planned abacus.

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