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Fo"cus
(), n.; pl. E. Focuses (), L. Foci (). [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.
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.
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 ellip
Fo"cus
(), v. t.[imp. & p. p.Focused (); p. pr. & vb. n.Focusing.] To bring to a focus; to focalize; as, to focus a camera. R. Hunt.
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