. Biophysical science. Biophysics. 2 : 3/ Light and the Eye 39 average human eye from the second principal point H' to the posterior focal point +F is cm. Thus, the eye has a strength of about 48 diopters. If the eye is stronger than this, images of distant objects will be focused in front of the retina. Such an eye is called near-sighted or myopic because near objects will be focused on the retina. This ocular defect can be corrected by placing a negative (diverging) lens in front of the eye. "Normal" vision is the ability to focus on the retina images of objects more than 25 cm
. Biophysical science. Biophysics. 2 : 3/ Light and the Eye 39 average human eye from the second principal point H' to the posterior focal point +F is cm. Thus, the eye has a strength of about 48 diopters. If the eye is stronger than this, images of distant objects will be focused in front of the retina. Such an eye is called near-sighted or myopic because near objects will be focused on the retina. This ocular defect can be corrected by placing a negative (diverging) lens in front of the eye. "Normal" vision is the ability to focus on the retina images of objects more than 25 cm away. If the refractive power of the eye is too weak, the image will be formed behind the retina, and positive lenses are needed for correction. Such eyes are called hyperopic or far-sighted. By and large, it is not possible to design a corrective positive lens for objects at all distances and so bifocals or trifocals are necessary. Another frequent defect, which can be corrected by glasses, is called astigmatism. This defect consists of having different focal lengths for lines in different directions. A so-called normal person would see all the lines of a fan chart, Figure 7, as equally black, whereas one with astigmatism will see lines in one meridian darker than those in the meridian at right angles. Astigmatism is due to the fact that some of the refractive surfaces of the eye, especially the cornea, are not spherical but have different curvatures in two meridians. To recapitulate, the eye lends itself to a description in the terms of geometrical optics. The eye is a system of spherical surfaces separated by media of different indices of refraction. Optically, it can be des- cribed in terms of six cardinal points. The common defects easily corrected by glasses can also be described in the language of geometrical Figure 7. Pattern for observ- ing astigmatism. C. Histology of the Eye Each gross structure of the eye can be described on a microscopic scale. This is th
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