Defects of sight and hearing : their nature, causes, prevention, and general management / by T. Wharton Jones.

  • Thomas Wharton Jones
Date:
1859
Catalogue details

Licence: Public Domain Mark

Credit: Defects of sight and hearing : their nature, causes, prevention, and general management / by T. Wharton Jones. Source: Wellcome Collection.

Provider: This material has been provided by the National Library of Medicine (U.S.), through the Medical Heritage Library. The original may be consulted at the National Library of Medicine (U.S.)

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    near objects, 1 >v causing convergence of the rayi of light before they enter the eye, bo that they may be more speed- ily brought to foci than thcv would otherwise 1', i ' quenoe of the diminished refractive power of the eyej or, supposing the retractive power of the eye not diminished, but the distance of the retina behind the lens diminished, that they may be brought to foci at a less distance behind the lens, than they would otherwise be, in order to corres- pond with the diminished distance of the retina behind the lens. Presbyopic persons, at the same time, that tiny ha\. the power of adjustment for near objects, may be somewhat oataractoUS, or have their retina impaired in sensihilit v. In such eases, convex glasses, besides remedying the defective adjustment, arc of use by concentrating the light. Convex glasses are made of different degrees of convexity. The least convex being those adapted for the slighter degrees of far-sightedness, the more convex for the greaterdeg To see distant objects, far-sighted persons do not in g( oeral require convex glasses. It is most commonly to enable them to read and do minute work that they use spectacles. If it is only at a very great distance that a person can sec distinctly, the focal length of the convex glass which he will require to enable him to read will be equal to the distance at which he wisl loread. [This paragraph will not apply to persons from 65 years and upwards.] If be is not so very far-sighted, but can see small ol distinctly at twenty inches distance, for example, the local length of the com bich he will requii enable him to read at twelve inches distance, is determined by multiplying the two distances together, and dividing