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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.

Hyperbola :: Hyperbola (n.) A curve formed by a section of a cone, when the cutting plane makes a greater angle with the base than the side of the cone makes. It is a plane curve such that the difference of the distances from any point of it to two fixed points, called foci, is equal to a given distance. See Focus. If the cutting plane be produced so as to cut the opposite cone, another curve will be formed, which is also an hyperbola. Both curves are regarded as branches of the same hyperbola. See Illust. o

Image :: Image (n.) The figure or picture of any object formed at the focus of a lens or mirror, by rays of light from the several points of the object symmetrically refracted or reflected to corresponding points in such focus; this may be received on a screen, a photographic plate, or the retina of the eye, and viewed directly by the eye, or with an eyeglass, as in the telescope and microscope; the likeness of an object formed by reflection; as, to see one's image in a mirror..

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..

Corradiate :: Corradiate (v. t.) To converge to one point or focus, as light or rays..

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.

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..

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

Collimator :: Collimator (n.) A tube having a convex lens at one end and at the other a small opening or slit which is at the principal focus of the lens, used for producing a beam of parallel rays; also, a lens so used..

Focusing :: Focusing (p. pr. & vb. n.) of Focu.

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

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..

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

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

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

Focalize :: Focalize (v. t.) To bring to a focus; to focus; to concentrate.

Hypermetropy :: Hypermetropy (n.) A condition of the eye in which, through shortness of the eyeball or fault of the refractive media, the rays of light come to a focus behind the retina; farsightedness; -- called also hyperopia. Cf. Emmetropia..

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..

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

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..

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