FOCUS, in ordinary language, any place from which an influence emanates, or where that influence exists in very concentrated form. In optics, a point at which the rays of light refracted from a convex lens, or reflected from a con cave mirror, are most concentrated; a point in which such rays meet, or tend to meet; if produced either backward or forward. In conic sections: (1) Singular (of a parabola) : A point so situated that if from it there be drawn a line to any point in the curve, and another from the latter perpendicular to a straight line given in position, these two straight lines will always be equal to one an other. (2) Plural: (a) Of an ellipse: Two points so situated that if two straight lines be drawn from them to any point in the curve, the sum of these straight lines will always be the same. (b) Of a hyperbola: Two points so situated that if two straight lines be drawn from them to any point in the curve, the excess of the straight line drawn to one of the points above the other will always be the same.
In astronomy, the term foci is often used in connection with the orbit of the earth, which is an ellipse, with the sun in one of the foci. In acoustics, the point of convergence of sound rays, these fol lowing the same laws as those of light and heat. Acoustic focus: The focus of sound rays. Calorific focus: The focus of heat rays. Conjugate foci: In optics, two foci so situated that, if rays of light diverging from one strike a concave mirror, they will be reflected and meet in the other. Luminous focus: In optics,
the focus of light rays. Principal focus: In optics, the focus of parallel rays strik ing a concave mirror. Vertical focus: In optics, a radiant point behind a mirror, from which rays may be held to diverge more and more, and in which, looking at them now as coming from the opposite direction, and consequently as conver gent, they would tend to meet. Magnetic foci: The two points on the earth's sur face where the magnetic intensity is greatest; they nearly coincide in position with the magnetic poles.
FOG, a very thick mist; small hollow vesicles of water suspended in the air, but so low as to be but a short distance from the earth in place of rising high above it and becoming so illuminated by the sun as to constitute clouds of varied hue. Fogs often arise when the air above warm, moist soil is colder than the soil itself. The hot vapors from the ground are then condensed by coming in contact with the colder air above, as the warm steam of a kettle is by the comparatively cold air of a room. But no fog arises till the cold air has absorbed vapor enough to bring it to the point of saturation. Fogs often hang over rivers. Their cause is the conden€ation by contact with the cold water, of the vapor in a hot and moist air current passing over the river.