Diseases of the eye : a treatise contributed to the second edition of Holmes's System of surgery / by James Dixon.

  • Dixon, James, active 1870.
Date:
1870
Catalogue details

Licence: Public Domain Mark

Credit: Diseases of the eye : a treatise contributed to the second edition of Holmes's System of surgery / by James Dixon. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

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    divergently. It is obvious, however, that the divergence of any rays falling on the eye must become less in proportion as their source is removed farther off; so that by the time the rays from a distant object are passing through the small aperture of the pupil, their divergence has become so trifling that, for all prac- tical purposes, they may be regarded as parallel. In considering the eye as an optical instrument, we have to do only with those rays of light which are admitted by the pupil—they alone being concerned in vision. While in a perfectly formed, and normally refractive, or emmetropic eye, the rays from distant objects would naturally come to a focus upon the retina, and there form a picture which is to be recognised by the sensorium, the rays from near objects, falling on the surface of the cornea in a state of more or less divergence, would, in passing on through the refractive media of the organ, tend towards a focus behind the retina. (Fig. 140.) To effect near vision, therefore, it becomes necessary that the media should undergo such a change as will bring divergent rays to an exact focus on the retinal surface. This change is the result of an active effort which is called Accommodation. Fig. 140. a ol Emmetropic eye in a state of rest. Parallel rays, a, a, come to a focus on the retina at b. Rays from a near object, c, being divergent, tend towards a focus behind the retina, d. The act of accommodation, which consists in increased convexity of the lens, causes these divergent rays to converge to a focus at b. Various theories have at different times been suggested to explain the mechanism of this act of adjustment or accommo- dation of the eye to near objects. First, it was said that by a contraction of the recti muscles the eyeball was compressed on all sides, and thereby elongated in its antero-posterior diameter, and thus the retina was actually removed backwards into such a position as enabled it to receive upon its surface the exact focus of the rays. Secondly, according to another theory, the act of accommodation consisted in an increased curvature of the cornea. This Home and Eamsden considered themselves to