Twenty minutes' advice on the eyes, and the means of preserving the sight ... together with the right time for using glasses, and the choice of them / By a retired oculist.

Date:
1834
Catalogue details

Licence: Public Domain Mark

Credit: Twenty minutes' advice on the eyes, and the means of preserving the sight ... together with the right time for using glasses, and the choice of them / By a retired oculist. Source: Wellcome Collection.

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    conformation, parallel rays form their foci before they arrive at the retina, then the person can see very near objects only. Such persons are said to be near¬ sighted, or they are called myopes. When the eye is flatter than ordinary, then the foci of rays from pretty near objects are formed beyond the retina. Persons with such eyes are called presbytoe ; they can adjust their eyes for objects beyond a certain distance only. The latter is generally the case with old persons; but the eyes of old persons sometimes are incapable of adjustment both for very near and for very distant objects. This comes from a rigidity or want of plia¬ bility in the parts. These defects are frequently brought on or increased by habit, as by the constant habit of viewing objects either from too near or from too great a distance, as also by the use of improper glasses. These imperfections may in a great measure be remedied by the use of proper glasses or spectacles ; for since in near-sighted persons the rays of light converge to a focus too soon, viz. before they come to the retina, concave lenses, which diminish the con- vergency, must remove the imperfection: and for those who can see distant objects only with tolerable distinctness, viz., in whose eyes the rays do not con¬ verge soon enough, those convex lenses, which increase the convergency, must remove the imperfection. When the defect comes from rigidity, as in some
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