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Improve Eyesight
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Improve Eyesight: Two Theories
Accommodation is the process by which the eye causes objects at various distances to focus on the retina. As with a camera, there are basically two ways to produce a clear image. One is to change the curvature of the lens doing the focusing. The other method is to change the distance between the lens and the focal plane.
This, in essence, expresses the two theories of how the eye accommodates.
Helmholtz Theory of Accommodation
In 1855 Hermann von Helmholtz performed experiments in which he moved a flame from one distance to another from seated subject. He had photographic equipment positioned to capture the reflection of the flame from of the front of the lens of his subject. As he brought the candle closer he noted that the curvature of the front of the lens increased.
Helmholtz concluded that the changing curvature of the lens was the eye's means of accommodation.
The curvature of the lens is controlled by the ciliary muscle that circles around the lens. When the ciliary muscle is relaxed the lens is relatively flat. When the ciliary muscle contracts the front of the lens becomes rounder, increasing the focusing power of the lens.
Show here is the shape of the lens according to Helmholtz. The left side shows the relaxed ciliary with the relatively flat front portion of the lens. During accommodation for near objects the ciliary muscle contracts and the front of the lens becomes more curved as shown on the right side.
Light rays from a distant object approach the eye so they are nearly parallel. The lens can be relatively flat to properly focus these rays on the retina. Light rays from nearby objects, however, approach the eye at an angle and the lens must have more curvature to properly focus these rays on the retina.
The Helmholtz theory has been adopted by the medical profession. When you do not focus clearly, the medical professional see the cause as a faulty lens shape and to improve eyesight prescribes artificial lenses in the form of eyeglasses or contact lenses or uses various techniques to reshape the cornea or the lens using techniques such as Lazik surgury.
Bates Theory of Accommodation
External to the eye, there are six muscles that control the position of the eye in the eye socket. Four of the muscles attach to the eye and extend backward. Two muscles partially curve around the eye.
William Bates, M.D. (1860-1931) performed numerous experiments that convinced him that accommodation was not produced by the ciliary muscles within the eye. Rather, he believed accommodation was produced by the external muscles of the eye.
Bates believed tension of these external muscles could elongate the eye or shorten the eye. It is the muscles that partially surround the eye that help elongate the eye and move the focal plane of the retina further from the lens as required for seeing nearby objects.
Bates concluded that nearsightedness or farsigntedness was caused by chronic tension in these external muscles. And, that by helping the patient to relax these muscles the focusing problems could be reduced or eliminated.
Chronic tension in the "rectus" muscles that extend backward from the eye pull the eye into the eye socket and shorten the eye; this produces farsightedness. Chronic tension in the "oblique" muscles that partially surround the eye cause the eye to elongate; this produces nearsightedness.
Those who hold the Bates theory help people with nearsightedness, farsightedness, astigmatism, presbyopia and other eye problems to improve eyesight by helping them learn to relax the eye's external muscles, reducing strain and restoring the eye's normal shapes.
For another insight into the Bates vs. Helmholtz controversy see my recommend resources page where I describe a fascinating discovery about the eye revealed in Relearning to See.
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Copyright © 2009 Robert Sherman