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

  • Thomas Wharton Jones
Date:
1859
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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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    in fact, the natural form of the cornea, hut in the insl referred fco, existing, perhaps, in an exaggerated degi as to disturb vision. With such unnatural conformation of the eye, I point appears a line of ■ certain length) a eiivle an <>val; every thing being Been elongated in one direction. The cylindri- cal deformation has been met with oblique, so thai a square appeared a parallelogram. The defect is remedied by glasses which, to the healthy would make a line of the .same length appear a point, —which would, in fact, shorten all objects in the .same de- gree and in the same direction, as they are lengthened by the defective eye.* [Several oases of oylindrioity have known to opticians in this country, and have been satisfactorily relieved by glasses of the above kind.] cylindrical eye being thus more or less pe- culiar, Lenses must be specially prepared for it; and it is evident that this demands both skill and intelligence on the part of the optician. The general principle on whioh the is Bhaped, Mr. Ross informs me, is this: one side of the lens is made a portion of a cylinder, of the same diam- eter BS the cylinder cornea, having its axis, however, placed at right angles to that of the latter. The other side ofthe lens is made plane, convex, or concave, to suit the condi- tion of the eye irrespective of its cylindricity. Conical Cornea. Defective sight sometimes occurs, in consequence of the cornea gradually becoming so prominent in the middle, as to be of a conical shape. Short-sightedness first at; * On the Use and Abase of Spectacles, by Andrew Rom, optician, London.