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e eye in which the rays of light are all brought accurately and without undue effort to a focus upon the retina; -- opposed to hypermetropia, myopia, an astigmatism.

Belonging to,or concerning, a focus; as, a focal point. Focal distance, or length,of a lens or mirror (Opt.), the distance of the focus from the surface of the lens or mirror, or more exactly, in the case of a lens, from its optical center. --Focal distance of a telescope, the distance of the image of an object from th…

The act of focalizing or bringing to a focus, or the state of being focalized.

To bring to a focus; to focus; to concentrate. Light is focalized in the eye, sound in the ear. De Quincey.

(Photog.) An assisting instrument for focusing an object in or before a camera. Knight.

ho perfected it in 1663. A small concave mirror in the axis of this telescope, having its focus coincident with that of the large reflector, transmits the light received from the latter back through a hole in its center to the eyepiece placed behind it. -- Gregorian year, the year as now reckoned according to the Greg…

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. of Conic section, and Focus.…

gh 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.

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…

er, or a white spot on a dark ground larger, than it really is, esp. when a little out of focus.

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.

al or in difference of pitch from another tone. Major axis (Geom.), the greater axis. See Focus, n., 2. -- Major key (Mus.), a key in which one and two, two and three, four and five, five and six and seven, make major seconds, and three and four, and seven and eight, make minor seconds. -- Major offense (Law), an off…

e angles. The measurement given directly is that of the image of the object formed at the focus of the object glass. Circular, or Ring, micrometer, a metallic ring fixed in the focus of the object glass of a telescope, and used to determine differences of right ascension and declination between stars by observations of…

Simple, or Single, microscope, a single convex lens used to magnify objects placed in its focus.

ightedness; a condition of the eye in which the rays from distant object are brought to a focus before they reach the retina, and hence form an indistinct image; while the rays from very near objects are normally converged so as to produce a distinct image. It is corrected by the use of a concave lens.…

eculiar shape reflects light from the entire horizon to the lens, by means of which it is focused upon the plate.

ides. 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.

displacement when the eye changes its position, caused by their not being exactly in the focus of the object glass. -- Stellar parallax, the annual parallax of a fixed star.

-- said of sets of eyepieces so mounted that they may be interchanged without varying the focus of the instrument (as a microscope or telescope) with which they are used.

Of certain achromatic lenses, having the same focus for the actinic and for the brightest of the visual rays.