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Encyclopedia Britannica - Main :: SOU-STE |
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SPECTACLES , the name given to flat glasses, prisms, spherical or cylindrical lenses, mechanically adjusted to the human eyes, so as to correct defects of vision (q.v.). They are made usually of crown glass or rock crystal (" pebbles "), the latter being somewhat lighter and cooler to wear
short-sight tends to increase during the early, especially the school, years of life, and that hygienic treatment, good light, good type, and avoidance of stooping are important for its prevention. Convex Lenses.In hypermetropia the retina is in front of the principal focus of the eye. Hence in its condition of repose such an eye cannot distinctly see parallel rays from a distance and, still less, divergent rays from a near object. The defect may be overcome more or less completely by the use of the accommodation. In the slighter forms no inconvenience may result; but in higher degrees prolonged work
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Anisonzetropia.It is difficult to lay down rules for the treatment of cases where the refraction of the two eyes is unequal. If only one eye is used, its anomaly should be alone corrected; where both are used and nearly of equal strength, correction of each often gives satisfactory results. Presbyopia.When distant vision remains unaltered, but, owing to gradual failure of the accommodative apparatus of the eye clear vision within 8 in. becomes impossible, convex lenses should be used for reading of such a strength as to enable the eye to see clearly about 8 in. distance. Presbyopia is arbitrarily said to commence at the age of forty
Cylindrical Lenses.In astigmatism, owing to differences in the refractive power of the various meridians of the eye, great
Convex spectacles were invented (see LIGHT) towards the end of the 13th century, perhaps by Roder Bacon. Concave glasses were introduced soon afterwards. Sir G. B. Airy, the astronomer, about 1827, corrected his own astigmatism by means of a cylindrical lens. Periscopic glasses were introduced by Dr W. H. Wollaston. End of Article: SPECTACLES If you wish, you can link directly to this article.
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