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The Eye as an Optical Instrument

المؤلف:  GEORGE A. HOADLEY

المصدر:  ESSENTIALS OF PHYSICS

الجزء والصفحة:  p-499

2025-12-23

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The eye is a minute camera, with dark chamber, lens, diaphragm, and screen upon which the image is formed. The sclerotic coat S (Fig. 1) forms the wall of the dark chamber, and is extended in front as a transparent coat C called the cornea. The lens L of the eye is formed 'of an elastic, transparent substance, and is called the crystalline lens. Extending over the front of the lens is a colored curtain or diaphragm, the iris, which determines the color of the eye, and which has a circular opening in the center called the pupil. The image of an object is brought to a focus upon the inner lining r of the eye called the retina, from which the sensation of sight is carried to the brain by means of the optic nerve. The yellow spot Y is the most sensitive part of the retina, and, since it is on the axis of the eye, is the spot on which is formed the image of the point at which the eye looks directly.

We have seen that in the camera the distance between the lens and the ground glass is changed in order to bring a given object to a focus. The distance between the crystalline lens and the retina, however, is a fixed distance, and the focus is obtained by a change in the curvature of the front of the lens, thus changing its focal length. This change of curvature of the lens to change its focus for objects at different distances is called accommodation.