The refraction of the eye : a manual for students / by Gustavus Hartridge.

  • Gustavus Hartridge
Date:
1894
Catalogue details

Licence: Public Domain Mark

Credit: The refraction of the eye : a manual for students / by Gustavus Hartridge. Source: Wellcome Collection.

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    the flame; the first upinght^ formed by the cornea; the second, larger, upright and formed by the anterior surface of the lens; the third smaller and inverted, formed by the posterior surface of the lens. When accommodation is put in force, images one and three remain unchanged in shape and position; image two, which is that formed by the anterior surface of the lens, becomes smaller, more distinct, and approaches image one, proving that this surface of the lens has become more convex and has approached the cornea. In an emmetropic eye adapted for infinity, it has been proved that the radius of curvature of the anterior surface of the lens is 10 mm.; when accommo- dated for an object 13'5 cm. off, it is changed to 6 mm. The pupil also becomes smaller, the central part of the iris advances, while the peripheral part slightly recedes. The alteration in the shape of the lens is due to the contraction of the ciliary muscle, which draws forward the choroid, thereby relaxing the suspensory ligament and allowing the elasticity of tlie lens to come into play. This elasticit}^ is due to the peculiar watch-spring arrangement of the lens fibres. When the ciliary muscle is relaxed, the suspensory ligament returns to its former state of tension, and so tightens the anterior part of the capsule, flattening the front surface of the lens. When the muscle is thus relaxed to its uttermost, the lens has assumed its least convexity, and the eye is then adapted for its far point {punctum remotum) {r).
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