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 . They are mounted in 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 is apt to give rise to aching and watering of the eyes, headache, inability to read or sew for any length of time, and even to double vision and 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 total hypermetropia is corrected . Young adults with slighter forms of hypermetropia need glasses only for near work; elderly people should have one pair of weak glasses for distant and another stronger pair for near vision . These may be conveniently combined, as in Franklin glasses, where the upper half of the spectacle frame contains a weak lens, and the lower half, through which the eye looks when reading, a stronger one .

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, because it is then that the need of spectacles for reading is generally 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 differences in the refractive power of the various meridians of the eye, 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 combination with spherical lenses . The correction of astigmatism is in many cases a matter of considerable difficulty, but the results to vision almost always reward the trouble . 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 .