Student's guide to diseases of the eye / by Edward Nettleship ; with a chapter on examination for color perception by William Thomson.

Nettleship, Edward, 1845-1913.

Date:: 1883

Credit: Student's guide to diseases of the eye / by Edward Nettleship ; with a chapter on examination for color perception by William Thomson. Source: Wellcome Collection.

Provider: This material has been provided by the Francis A. Countway Library of Medicine, through the Medical Heritage Library. The original may be consulted at the Francis A. Countway Library of Medicine, Harvard Medical School.

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$clearly; and additional accommodation will give also dis- tinct vision of near objects (compare Fig. 89). A little consideration will show that the competence of the ciliary muscle to give these results will depend in any given case: (1) on the degree of advancement of the retina in front of 11} permetropia corrected by accommodation. Parallel rays focussed tu retina. f, i. e., on the degree of shortening of the eye; and (2) on the strength of A., i. e., on the extent to which the focal length of the lens can be shortened. Fig. 97 may be taken for a section of a very highly hypermetropic eye, the rays emerging from which are Fig. 97. Course of the rays emerging from a hypermetropic eye. divergent. The image formed on the retina of a hyper- metropic eye is smaller than that of the same object placed at the same distance from a normal eye (p. 25, § 19). In old age the crystalline lens becomes flatter and less refractive, and, therefore, an eye originally emmetropic is now unable to focus parallel rays on the retina; this con-$