Nettleship, Edward, 1845-1913 · Philadelphia ; New York : Lea brothers & co. · leaf 31
irected toward the axis will be focussed at c" /", because the angle a" — a; no real conjugate to c" f" exists; but if the ray start fron c" f" it will, on taking the direction c , appear to have come from vf which consequently is the virtual focus of c" f", see § 6. 14. All the foci of concave… … Foci of a concave lens. 15. In equally biconvex or biconcave lenses of crown glass the principal focus f is at the centre of curvature of either surface of the lens — i. e., f =r, the radius ; in plano¬ convex, or concave, lenses/— 2r. 10. Images. The image formed by a lens consists of foci, each…
Eye -- Diseases · Color blindness
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