Licence: In copyright
Credit: General and practical optics / by Lionel Laurance. 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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![form B is employed and D3 disappears. Thus for a plano-cyl. of form A having = - 1-5, and 0^ = 0, the being 1-52 and 1-65, we find Ox (1-52-l)-[-1-5 (1-65-1)] +-975_^ . 1-65-1-52 ^ ^ •13 ''^''^ Let Dj= + -5 and +2*25, made periscopic so that i-l*25 and D5 = — 75 ; using form (D) we get [ + 2-25-(--75)] •52-(U25 x-65) '7475 1-65-1-52 ~ -13 ~ ''^ If the /x's are 1-52 and 1-65, D = 4 (D2-D5)-5 D3 for forms 6', D and ^; D = 4 D2-5 D3 for form A; D = 4 (Dg- D5) for form B. If the /x's are 1-5 and 1*6, D = 5 (Dg —D^) —6 D3 for forms C, and E; D = 5 D2-6 D3 for form A; D = 5 (Dg-D5) for form B. The disc selected must be rather thicker than for ordinary lenses, especi- ally if the segment is of high power. Having insets of known powers the selection of a suitable blank and the curvatures of the two outer surfaces are as follows. Let the two /x's be 1*65 and 1*52 so that for a given curva- ture producing D3 we have D^ = 5 ^3/4, i.e. is 1/4 stronger than D3, and J)^ — J)^ = DJ^. Now part of the additional power for reading is obtained from D4 —D3, and part from D^3, i.e. \)^ — D^= (D^ —D3) + Dg; therefore the powers needed on the two surfaces are D3 = 4 (D2 — Di — Dg), and D5 = Dj^ — D3. It is preferable to select a disc such that Dg is higher than the addition needed for reading, and in that case D3 is Cc. if D^^ is Cx. In all cases it is advisable to calculate two or three combinations in order to arrive at the most suitable. A Cc. curvature on the surface of should be avoided ; other- wise there is danger of working through to the segment. If Dg = Dg — the surface D3 is piano; therefore for sph.-cyls. select Dg = D2 —l*25Dj, the cyl. being ground on to the side of Dg, and the sph. on that of D3. The proportional increase of power of over D3 is found from (/^2-/^i)-(/^i-l) (/-i-l) so that if the two /x's are other than those given above, the factor 4 in the value of D3 would vary accordingly. As examples, for D-^=^+'22b, and D2=-l-3*5 select Dg=l'5; then D3 = 4x (1-25-1-5)= -1, and 0^ = 2-25 + 1 = +3-25. For Di= -3-5,andD2= -2-25 select Dg = 2-5 ; thenD3 = 4x (1-25-2-5) = - 5 and T>^= -3-5 + 5 = 1-5. For +6 S. O -2 C. with +8 sph. for reading, Dg= +8-1-25 x 6 = -5. For -10 S. O -3 C. with -7 sph. for reading Dg=—7 + 1*25 x 10=5-5. To Construct Test Types after Snellen.—Each letter at a certain distance](https://iiif.wellcomecollection.org/image/b21287946_0357.jp2/full/800%2C/0/default.jpg)
