. Library of universal knowledge. A reprint of the last (1880) Edinburgh and London edition of Chamber's encyclopaedia, with copious additions by American editors. hrough it, andform an inverted image smaller or largerthan the object according as the object isat a distance greater or less than twice the principal focal length; but the image is erectand magnified if the object be within the principal focal length. For a concave lens thefocus is virtual—the rays seem to come from it and form an erect image smaller thanthe object. The following is the mode of finding the piincipal focus when para
. Library of universal knowledge. A reprint of the last (1880) Edinburgh and London edition of Chamber's encyclopaedia, with copious additions by American editors. hrough it, andform an inverted image smaller or largerthan the object according as the object isat a distance greater or less than twice the principal focal length; but the image is erectand magnified if the object be within the principal focal length. For a concave lens thefocus is virtual—the rays seem to come from it and form an erect image smaller thanthe object. The following is the mode of finding the piincipal focus when parallel rays fall on adouble convex lens (Fig. 2): O is the center of the curved surface PAP, and O of thesurface PBP; q is the point towards winch the raystend while passing through the lens, and F the pointto which they converge after emergence. Let 0A=r, 0B=/;. A7=/, and BF (the focal length) =/;then neglecting the thickness of-the lens, which mayl)e done when the curvature of the lens is small, A^=Br/, and AF = BF. By the demonstration given under the article Dioptrics, we find/ = —^-- r, for the refraction at the first surface;U. K. YIIL—51 —-^^ ^. Fio. 1.—, double-conves^; if tbe surfaces are of equal , equi-convex; 2, plano-convex; 3, convexo-plane; 4, double-concave, or concavo-concave; 5,f)lano-concave; 0. concavo-plane; 7, convex-menis-cus; 8, concave-menii-cus; 9, convexo-concave; 10,concavo-convex. The arrow shows the direction inwhich the light is supposed to fall.
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Keywords: ., bookcentury1800, bookdecade1880, bookpublishernewyorkamericanboo
