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.)

    24/260
    -0 DXROM Of sight. the eye in a state of divergence from the several pointi of an externa] object, thai they arc made to oonvergc to points or foci. These points or foci of light impinge on the retina, as on the table of a camera obsoura, in the same relative order as that in which tlie rayi forming them originally diverged from the several points of the externa] object. As to the particular action of the dioptric parts of the eye on the rays of light in their course to the retina : of the pencils of rays diverging from the various points of external objects, those only which fall on the cornea, and are transmitted by it to the aqueous humor, and thenoe through the pupil to the retina, are concerned in vision. Those rays which fall on and pass through the circumferen- tial pari of the cornea, arc stopped by the iris, ami are either reflected or absorbed by it. The effect of the refrac- tion by the cornea, and of that by the different humors on the rays, is to make them converge; hence, each pencil represents two cones, with their bases touching each other at the anterior surface of the cornea, fig. 1, p. 14. Of all the dioptric parts of the eye, the cornea is that which contributes most to the convergence of the rays of light. This is because the rays enter it from the air, which is of so much less refractive density than the cornea. The crystalline lens, though double convex, and of greater refractive density than the cornea, contributes comparative- ly little to the convergence of the rays of light, because the latter enter the crystalline lens from the aqueous bumor, the refractive density of which is not so much exceeded by that of the crystalline as the refractive density of the air is by that of the cornea. The aqueous humor being some- what less dense than the cornea, aud presenting a convex