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SPECTACLES

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Originally appearing in Volume V25, Page 618 of the 1911 Encyclopedia Britannica.
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SPECTACLES , the name given to See also:

flat glasses, prisms, spherical or cylindrical lenses, mechanically adjusted to the human eyes, so as to correct defects of See also:vision (q.v.). They are made usually of See also:crown See also:glass or See also:rock crystal (" pebbles "), the latter being somewhat lighter and cooler to See also:wear. They are mounted in See also:short-sight tends to increase during the See also:early, especially the school, years of See also:life, and that hygienic treatment, See also:good See also:light, good type, and avoidance of stooping are important for its prevention. See also:Convex Lenses.—In hypermetropia the retina is in front of the See also:principal See also:focus of the See also:eye. Hence in its See also:condition of repose such an eye cannot distinctly see parallel rays from a distance and, still less, divergent rays from a near See also:object. The defect may be overcome more or less completely by the use of the See also:accommodation. In the slighter forms no inconvenience may result; but in higher degrees prolonged See also:work is See also:apt to give rise to aching and watering of the eyes, headache, inability to read or sew for any length of See also:time, and even to See also:double vision and See also:internal strabismus. Such cases should be treated with convex lenses, which should be theoretically of such a strength as to fully correct the hypermetropia. Practically it is found that a certain amount of hypermetropia remains latent, owing to spasm of the accommodation, which relaxes only gradually. At first glasses may be given of such a strength as to relieve the troublesome symptoms; and the strength may be gradually increased till the See also:total hypermetropia is corrected. See also:Young adults with slighter forms of hypermetropia need glasses only for near work; elderly See also:people should have one pair of weak glasses for distant and another stronger pair for near vision. These may be conveniently combined, as in See also:Franklin glasses, where the upper See also:half of the spectacle See also:frame contains a weak See also:lens, and the See also:lower half, through which the eye looks when See also:reading, a stronger one.

Anisonzetropia.—It is difficult to See also:

lay down rules for the treatment of cases where the See also:refraction of the two eyes is unequal. If only one eye is used, its See also: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 See also: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 See also:age of See also:forty, because it is then that the need of spectacles for reading is generally See also:felt; but it appears later in myopia and earlier in hypermetropia. It advances with years, requiring from time to time spectacles of increasing strength. Cylindrical Lenses.—In astigmatism, owing to See also:differences in the refractive See also:power of the various meridians of the eye, See also:great defect of sight, frequently accompanied by severe headache, occurs. This condition may be cured completely, or greatly improved, by the use of lenses whose surfaces are segments of cylinders. They may be used either alone or in See also:combination with spherical lenses. The correction of astigmatism is in many cases a See also:matter of considerable difficulty, but the results to vision almost always See also:reward the trouble. Convex spectacles were invented (see LIGHT) towards the end of the 13th See also:century, perhaps by Roder See also:Bacon. See also:Concave glasses were introduced soon afterwards. See also:Sir G.

B. See also:

Airy, the astronomer, about 1827, corrected his own astigmatism by means of a cylindrical lens. Periscopic glasses were introduced by Dr W. H. See also:Wollaston.

End of Article: SPECTACLES

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