Appleton's dictionary of machines, mechanics, engine-work, and engineering . at they either converge,tending to a point beyond the lens, or diverge, as if they proceeded from a point before the lens; orbecome parallel, after converging or diverging. A double convex lens, Fig. 2581, is bounded by two convex spherical surfaces, whose centres are onopposite sides of the lens. It is equally convex when the radii of both surfaces (that is, the distancesfrom the centres to the circumferences of the circle they belong to) are equal, and unequally convexwhen their radii or distances are unequal. A pla
Appleton's dictionary of machines, mechanics, engine-work, and engineering . at they either converge,tending to a point beyond the lens, or diverge, as if they proceeded from a point before the lens; orbecome parallel, after converging or diverging. A double convex lens, Fig. 2581, is bounded by two convex spherical surfaces, whose centres are onopposite sides of the lens. It is equally convex when the radii of both surfaces (that is, the distancesfrom the centres to the circumferences of the circle they belong to) are equal, and unequally convexwhen their radii or distances are unequal. A plano-convex lens, Fig. 2582, is bounded by a plane surface on one side, and by a convex one onthe other. A double concave lens, Fig. 25S3, is bounded by two concave spherical surfaces whose centres are onopposite sides of the lens. A plano-concave lens, Fig. 2584, is bounded by a plane surface on one side, and a concave one onthe other. A meniscus, Fig. 2585, is bounded by a concave and a convex spherical surface; and these two sur-faces meet, if continued. The focal distance, or distance of the focus from the surface of the lens, depends both upon the formof the lens and of the refractive power of the substance of which it is made ; in a glass lens, both sidesof which are equally convex, the focus is situated nearly at the centre of the sphere of which the surfaceof the lens forms a portion; it is at the distance, therefore, of the radius of the sphere. Fig. 2587. Plano-convex lens and rays converging.—Fig. 2586. Lenses that have one side fiat and the otnerconvex, (plano-convex,) have their focus at the distance of the diameter of a sphere, of which the convexsurface of the lens forms a portion, as represented in the figure. According to some opticians, the greatest diameter of a lens is half an inch; if it exceed that thicknessthey do not call it a lens, but a lenticular glass. Lenses are made either by blowing or lenses are small globules of glass melte
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Keywords: ., bookcentury1800, booksubjectmechanicalengineering, bookyear1861
