The theory of ocular defects and of spectacles / translated from the German of Hermann Scheffler by Robert Brudenell Carter with prefatory notes and a chapter of practical instructions.
- Hermann Scheffler
- Date:
- 1869
Licence: Public Domain Mark
Credit: The theory of ocular defects and of spectacles / translated from the German of Hermann Scheffler by Robert Brudenell Carter with prefatory notes and a chapter of practical instructions. Source: Wellcome Collection.
Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.
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![Fig. c. 1 central line, and g ii, l m be drawn at equal distances from c and parallel to n' n, and the chords L G, m h be produced to meet in i, we have the figure c, which represents a smaller prismatic lens G h p m l p' cut out eccentrically from a larger lens D d'. Then, since p n is snpposed appi'oxi- mately equal to p' n', the chords h m, l g may be considered as parallel to the tan- gents at p and p' ; the prism L i M will have the same refracting angle at i as that in § 2, and the deflection of the ray Q p p' q will not be altered. Hence it may be shown, as in that sec- ^ tion, that, if f c = e, e. I ~ + ~ ] = m; e ^ . m; V ?• r'J ?• fl- r as in p. 8 ; and, the other two formula; in that page having been already proved, we have d ^ ~Y ■ This lens, though made of a continuous piece of glass, evidently consists, m foi-m, of a prism interposed between two plano-convex lenses. The reader can easily conceive, or draw for himself, the figure representing a prism interposed betAveen two plano-concave lenses, where the base of the y^’ism will be turned fi'om the centre of the larger lens, and its sides will be tangents to its surfaces at p and p'. i ; \D \\ I'f \ ^ N' F N It will be seen from this investigation, that any eccentric portion of a lens must always combine the deflecting proper- ties of a prism, with the focalizing properties due to its own • curved surfaces.](https://iiif.wellcomecollection.org/image/b2236397x_0021.jp2/full/800%2C/0/default.jpg)
