Ophthalmic lenses : dioptric formulae for combined cylindrical lenses, the prism-dioptry and other optical papers / by Charles F. Prentice.

Prentice, Charles F. (Charles Frederick), 1854-

Date:: 1907

Credit: Ophthalmic lenses : dioptric formulae for combined cylindrical lenses, the prism-dioptry and other optical papers / by Charles F. Prentice. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

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$Fig. 36. In Fig. 36 two plano-toric lenses L1 and L- are shown to have the same toric surface as in Fig. 35. As the principal meridians in each of these lenses are convex and concave, we may write their formulae as follows: (CJ [— 3D. Ref. 90° 3 + 2D. Ref. 180°] Tor.(L1) (Dx) [+3D.Ref. 90° C — 2D. Ref. 180°] Tor.(L2) so as to distinguish them from the correlative formulae for crossed cylin¬ ders of identical refraction: (C3) — 3 cyl. axis 180° 3 + 2 cyl. axis 90° (D2) + 3 cyl. axis 180° 3 — 2 cyl. axis 90° and their sphero-cylindrical equivalents, which are respectively: j + 2 sph. 3 — 5 cyl. axis 180°. (C3) | — g sph. 3 _(- 5 cyl. axis 90°. ( — 2 sph. 3 + 5 cyl. axis 180°. (D3) | _j_ 3 sph. 3 — 5 cyl. axis 90°. The student should practise transforming optionally chosen formulas hy applying the rules given in § 34 and § 39, when he may refer to the appended tables to verify the correctness of his own work.$