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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    the mere age of the person, there ie ;i certain av< relation between the age and the focal length of the convex - required, which may be expressed as follows: B jean . . 40,48 70,75,80,85,90,100 1 length in inchi L6,1 i, LS, l. 9, B, 7. [We more commonly commence with glasses of forty- eight inches focal distance at the age of 40 to 15, and ;n the same ratio.] (Masses of a low power are popularly called preservers, and are sometimes had recourse to without any necessity, as if they possessed some specific quality preservative of the Bight. The truth is, to good eyes they will prove not but deprav< rs. Reading ;//ass.—This is B double convex lens, broad enough to permit both eyes to see through it. It is ased for the purpose of magnifying the object,—the names on maps, or the like,—whereas, convex speetacles are used merely to render objects distinct at a given distance, with- out magnifying them as above mentioned. Cataract Glasses. The difference in refractive power between the air and the cornea, being niueli greater than between the squeOUS humor and crystalline body, the greatest amount of re- fraction which the rays of light undergo in the eye, in order that they may converge to loci on the retina, is that effected by the cornea on their first entrance. The talliue body contributes comparatively little to the con- ney. Hence, vision after a successful operation for cataract, may be tolerably distinct for objects at a certain distance. .Still, in order, that it may \»perfectly distinct, the use of convex g required.