Elements of natural philosophy (Volume 2-3) . 204 NATURAL PHILOSOPHY. Double convex § 45. As an example, let us take the double convex lens taken as an -i . lens. example; Equation (33), is Generalequation; / ff JL 1 F. f n Value forreciprocal ofprincipal focaldistance; and, Equation (30), and Table (A\ m — 1 F. u — m—l(- + _), and as long as m > 1, we shall have, Equation fordiscussion; / // + f n (40) Fig. 27. For -__ > —, or / > Fit, f will be negative, and Fu f the vergency ofthe deviated Keal radiants rayS w[\l \)q ne-between . rp, principal focus g^tlVe. liiat and infinity;

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Elements of natural philosophy (Volume 2-3) . 204 NATURAL PHILOSOPHY. Double convex § 45. As an example, let us take the double convex lens taken as an -i . lens. example; Equation (33), is Generalequation; / ff JL 1 F. f n Value forreciprocal ofprincipal focaldistance; and, Equation (30), and Table (A\ m — 1 F. u — m—l(- + _), and as long as m > 1, we shall have, Equation fordiscussion; / // + f n (40) Fig. 27. For -__ > —, or / > Fit, f will be negative, and Fu f the vergency ofthe deviated Keal radiants rayS w[\l \)q ne-between . rp, principal focus g^tlVe. liiat and infinity; is to Say, if a wave proceedfrom a point Give real foci, upon the axis in front of the lens between the limits Fn,the principal focus, and infinity, it will be concentratedafter deviation, into a point upon the same line behind,and the focus will be .bor ^ —, or ii f<Fu,f will be positive, and the ver- Beai radiants gency of the deviated Avitliin the ipai focus; W1^ be is, if the wave pro-ceed from a point infront and situated be- prmcij Fig. 28.

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