SPECTACLES (Lat. spectaculum, show, from spectarc, to behold, from spccere, to see; con nected with Gk. skeptesthai, to look, Skt. spa.4, to see, OH0. spehon, Ger. spiihen, to spy). A device for the purpose of aiding the sight when impaired by age or otherwise. (See SIGHT, DEFECTS OF.) They were invented during the thirteenth century. The credit is attributed by some to Alessandro di Spina, a Florentine monk, and by others to Roger Bacon. The lenses are nearly always made of the best optical glass, and by the best makers are ground with extreme care. Lenses may be mounted in either eye-glass or spectacle frames, care being taken that the centre of tho lens is opposite the pupil and that the glasses are tilted at a proper angle for read ing or distance as desired. When separate lenses are required for these purposes the so-called bifocal glasses may be used. They are made by cementing a small strong lens upon the lower part of the other, so that when the eyes are low ered, as in reading, the line of vision passes through that portion. Lenses are now numbered, with one of a focus of one meter as a unit, known as one diopter. One with four times that
strength has a focal distance of one-quarter of a meter and is known as 4 D. (diopters). Con vex spherical lenses are used in hypermetropia, presbyopia, and after removal of the crystalline lens for cataract. Concave spherical lenses are employed in myopia. If astigmatism exists a cylindrical lens, either concave or convex as re quired, is used. As the astigmatism may be in only one meridian, while that at right angles to it is normal, or as there may be either hyperopic or myopic astigmatism in both, or hyperopic in one and myopic in the other, it is sometimes necessary to employ cross-cylinder lenses, a com bination of two cylinders with their axes at right angles to each other. Prismatic glasses are used in cases with weakness of ocular muscles, one being chosen which will make the images seen by the two eyes coincide. Combinations of cylindrical and spherical lenses and prisms are needed when errors of refraction or accommoda tion are combined with astigmatism and mum", lar weakness, which cause double vision.