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Noah Webster's 1828 American Dictionary
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It is not only important, but, in a degree necessary, that the people of this country, should have an American Dictionary of the English language; for, although the body of the language is the same as in England, and it is desirable to perpetuate that sameness, yet some differences must exist. Language is the expression of ideas; and if the people of one country cannot preserve an identity of ideas, they cannot retain an identity of language. |
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1828 Definition
FO'CUS, n. plu. focuses, or foci. [L. focus, a fire, the hearth.]
1913 Definition
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.
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 ellipse and hyperbola have each two foci, and two corresponding directrixes, and the parabola has one focus and one directrix. In the ellipse the sum of the two lines from any point of the curve to the two foci is constant; that is: AG+GB=AH+HB; and in the hyperbola the difference of the corresponding lines is constant. The diameter which passes through the foci of the ellipse is the major axis. The diameter which being produced passes through the foci of the hyperbola is the transverse axis. The middle point of the major or the transverse axis is the center of the curve. Certain other curves, as the lemniscate and the Cartesian ovals, have points called foci, possessing properties similar to those of the foci of conic sections. In an ellipse, rays of light coming from one focus, and reflected from the curve, proceed in lines directed toward the other; in an hyperbola, in lines directed from the other; in a parabola, rays from the focus, after reflection at the curve, proceed in lines parallel to the axis. Thus rays from A in the ellipse are reflected to B; rays from A in the hyperbola are reflected toward L and M away from B. A central point; a point of
concentration.
Aplanatic focus. (Opt.) See under Aplanatic. -- Conjugate focus (Opt.), the focus for rays which have a sensible divergence, as from a near object; -- so called because the positions of the object and its image are interchangeable. -- Focus tube (Phys.), a vacuum tube for Rœntgen rays in which the cathode rays are focused upon the anticathode, for intensifying the effect. -- Principal, or Solar, focus (Opt.), the focus for parallel rays. To bring to a focus] to focalize; as, to focus
a camera.
R. Hunt. | ||||||||
The second vision, Vision::Redesign, has an intent to improve the current online accessibility to the 1828 dictionary. The current database, http://1828.mshaffer.com/, has mistakes, omissions of etymology, etc. For this reason the most important tasks, Task::XML, will be an online editing of the current words and definitions to include important missing information. An administrative tool has been built to make this possible [http://1828d.mshaffer.com/] and will be accessible to the community to help in the editing process once the microfilm scan occurs.
