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contact lens
contact lens thin plastic lens worn between the eye and eyelid that may be used instead of eyeglasses. Actors, models, and others wear them for appearance, and athletes use them for safety and convenience. Contact lenses may also be used to correct certain abnormalities of the eye that cannot be co...
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nearsightedness
nearsightedness or myopia, defect of vision in which far objects appear blurred but near objects are seen clearly. Because the eyeball is too long or the refractive power of the eye's lens is too strong, the image is focused in front of the retina rather than upon it. Corrective eyeglasses wi...
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optician
optician filler of prescriptions for and dispenser of corrective lenses. An optician may grind lenses as instructed by the prescription of an optometrist (see optometry ) or ophthalmologist (see ophthalmology ) or transcribe the instructions for laboratory mechanics. The optician also fits and ad...
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cryosurgery
cryosurgery , bloodless surgical technique using a supercooled probe to destroy diseased or superfluous tissue. Liquid nitrogen circulating through the instrument cools it to temperatures as low as -196°C (-321°F). Tissue destroyed on contact with the probe is removed by phagocytic white blo...
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spinel
spinel magnesium aluminum oxide, MgAl 2 O 4 , a mineral crystallizing in the isometric system, usually as octahedrons. It occurs as an accessory mineral in basic igneous rocks, in aluminum-rich metamorphic rocks, and in contact-metamorphosed limestones. Common spinel usually ranges in color from da...
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relay
relay electromechanical switch operated by a flow of electricity in one circuit and controlling the flow of electricity in another circuit. A relay consists basically of an electromagnet with a soft iron bar, called an armature, held close to it. A movable contact is connected to the armature in ...
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rheostat
rheostat , device whose resistance to electric current depends on the position of some mechanical element or control in the device. Typically a rheostat consists of a resistance element equipped with two contacts, or terminals, by which it is attached to a circuit: a fixed contact at one end and a s...
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astigmatism
astigmatism , type of faulty vision caused by a nonuniform curvature in the refractive surfaces—usually the cornea, less frequently the lens—of the eye. As a result, light rays do not all come to a single focal point on the retina. Instead, some focus on the retina while others focus in ...
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optometry
optometry , eye-care specialty concerned with eye examination, determination of visual abilities, diagnosis of eye diseases and conditions, and the prescription of lenses and other corrective measures. In most states optometrists may prescribe drugs for the treatment of eye diseases. The principal c...
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aureole
aureole , in physics, luminous circle seen when the sun or other bright light is observed through a diffuse medium, i.e., smoke, thin cloud, fog, haze, or mist. It sometimes occurs as a series of concentric circles. The aureole results from the dispersion of light by particles of dust or water. Beca...
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