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Focus :: Focus (n.) A point so related to a conic section and certain straight line called the directrix that the ratio of the distace between any point of the curve and the focus to the distance of the same point from the directrix is constant.

Parabola :: Parabola (n.) A kind of curve; one of the conic sections formed by the intersection of the surface of a cone with a plane parallel to one of its sides. It is a curve, any point of which is equally distant from a fixed point, called the focus, and a fixed straight line, called the directrix. See Focus..

Focimeter :: Focimeter (n.) An assisting instrument for focusing an object in or before a camera.

Camera Obscura :: Camera obscura () An apparatus in which the images of external objects, formed by a convex lens or a concave mirror, are thrown on a paper or other white surface placed in the focus of the lens or mirror within a darkened chamber, or box, so that the outlines may be traced..

Focus :: Focus (v. t.) To bring to a focus; to focalize; as, to focus a camera..

Focal :: Focal (a.) Belonging to,or concerning, a focus; as, a focal point..

Diaphragm :: Diaphragm (n.) A plate with an opening, which is generally circular, used in instruments to cut off marginal portions of a beam of light, as at the focus of a telescope..

Directrix :: Directrix (n.) A straight line so situated with respect to a conic section that the distance of any point of the curve from it has a constant ratio to the distance of the same point from the focus.

Sunglow :: Sunglass (n.) A convex lens of glass for producing heat by converging the sun's rays into a focus.

Polar :: Polar (n.) The right line drawn through the two points of contact of the two tangents drawn from a given point to a given conic section. The given point is called the pole of the line. If the given point lies within the curve so that the two tangents become imaginary, there is still a real polar line which does not meet the curve, but which possesses other properties of the polar. Thus the focus and directrix are pole and polar. There are also poles and polar curves to curves of higher degree th

Ellipse :: Ellipse (n.) An oval or oblong figure, bounded by a regular curve, which corresponds to an oblique projection of a circle, or an oblique section of a cone through its opposite sides. The greatest diameter of the ellipse is the major axis, and the least diameter is the minor axis. See Conic section, under Conic, and cf. Focus..

Eccentricity :: Eccentricity (n.) The ratio of the distance between the center and the focus of an ellipse or hyperbola to its semi-transverse axis.

Reticule :: Reticule (n..) A system of wires or lines in the focus of a telescope or other instrument; a reticle.

Convergent :: Convergent (a.) tending to one point of focus; tending to approach each other; converging.

Irradiation :: Irradiation (n.) The apparent enlargement of a bright object seen upon a dark ground, due to the fact that the portions of the retina around the image are stimulated by the intense light; as when a dark spot on a white ground appears smaller, or a white spot on a dark ground larger, than it really is, esp. when a little out of focus..

Micrometer :: Micrometer (n.) An instrument, used with a telescope or microscope, for measuring minute distances, or the apparent diameters of objects which subtend minute angles. The measurement given directly is that of the image of the object formed at the focus of the object glass..

Amasthenic :: Amasthenic (a.) Uniting the chemical rays of light into one focus, as a certain kind of lens; amacratic..

Aberration :: Aberration (n.) The convergence to different foci, by a lens or mirror, of rays of light emanating from one and the same point, or the deviation of such rays from a single focus; called spherical aberration, when due to the spherical form of the lens or mirror, such form giving different foci for central and marginal rays; and chromatic aberration, when due to different refrangibilities of the colored rays of the spectrum, those of each color having a distinct focus..

Focuses :: Focuses (pl. ) of Focu.

Latus Rectum :: Latus rectum () The line drawn through a focus of a conic section parallel to the directrix and terminated both ways by the curve. It is the parameter of the principal axis. See Focus, and Parameter..

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