SPECTACLES, magnifying glasses made as a pair to be fitted over the eyes to aid the vision, supposed to have been invented by Roger Bacon in the 13th century, and used to magnify an object or correct some defect in the organs of vision. Spectacles consist generally of two oval or circular lenses mounted in a light metal frame which is made up of the "bows," °bridge," and "sides." The lenses may be bi concave, bi-convex, concavo-convex,etc., though lenses forming segments of a cylinder are used in some cases of astigmatism. Iii long-sighted persons the defect of the eye is counteracted by convex lenses, in short-sighted persons by concave lenses. When made with out bows to hold on the ears or hair, and with a spring in the bridge, they are specifically termed eye-glasses, or pince-nez.
Divided or pantascopic spectacles have each lens composed of two semi-circles of different foci neatly united one above the other; one half for looking at distant objects, and the other for examining things near the eye. An other kind, called periscopic spectacles, has been contrived in order to allow considerable lati tude of motion to the eyes without fatigue. In
such case the concave side of a diverging me niscus lens is placed toward the eye. A mod ern form is made of plain glass with large rims, as of imitation tortoise shell, for protec tion of the eyes against sand, etc., as encoun tered in driving an automobile. Colored lenses of flat glass are employed in spectacles to re lieve the eyes of a glare of light. Wings or °blinders" are sometimes added to keep out the dust. Where two lenses are supplied for each eye they are termed compound spectacles or compound lens spectacles. Ordinary stand ard spectacle lenses are numbered according to the inches of focal distance, a No. 12 involv ing holding a paper at a distance of 12 inches from the eye to get the correct focus and read easily. Astigmatism (q.v.) is so common that spectacles are frequently made to a prescrip tion, each glass being ground to a curve to cor rect the inaccuracy of the eye. Consult Crookes,