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Figure 49-15

Chart composed of parallel black bars at different angular orientations for determining the axis of astigmatism.

Figure 49-16

Maximum visual acuity for two point sources of light.

Figure 49-16

Maximum visual acuity for two point sources of light.

that the tears between the contact lens and the cornea have a refractive index almost equal to that of the cornea, so that the anterior surface of the cornea no longer plays a significant role in the eye's optical system. Instead, the outer surface of the contact lens plays the major role. Thus, the refraction of this surface of the contact lens substitutes for the cornea's usual refraction. This is especially important in people whose eye refractive errors are caused by an abnormally shaped cornea, such as those who have an odd-shaped, bulging cornea—a condition called keratoconus. Without the contact lens, the bulging cornea causes such severe abnormality of vision that almost no glasses can correct the vision satisfactorily; when a contact lens is used, however, the corneal refraction is neutralized, and normal refraction by the outer surface of the contact lens is substituted.

The contact lens has several other advantages as well, including (1) the lens turns with the eye and gives a broader field of clear vision than glasses do, and (2) the contact lens has little effect on the size of the object the person sees through the lens, whereas lenses placed 1 centimeter or so in front of the eye do affect the size of the image, in addition to correcting the focus.

Cataracts

"Cataracts" are an especially common eye abnormality that occurs mainly in older people. A cataract is a cloudy or opaque area or areas in the lens. In the early stage of cataract formation, the proteins in some of the lens fibers become denatured. Later, these same proteins coagulate to form opaque areas in place of the normal transparent protein fibers.

When a cataract has obscured light transmission so greatly that it seriously impairs vision, the condition can be corrected by surgical removal of the lens. When this is done, the eye loses a large portion of its refractive power, which must be replaced by a powerful convex lens in front of the eye; usually, however, an artificial plastic lens is implanted in the eye in place of the removed lens.

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Essentials of Human Physiology

Essentials of Human Physiology

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