BIFOCAL LENSES

composite lenses composed either of two lenses of different curvatures, or of one lens ground so that a small portion has a curvature differing from that of the remainder, used to correct vision in cases where different corrections must be applied for far and near vision. For example, in aphakia the absence of the crystalline lens of the eye, due either to dislocation, injury or removal by operation. a strong convex lens is needed for distant, and a still stronger one for near vision, and the upper portion of the bifocal lens prescribed will have a longer focal length than the small lower portion. In presbyopia, and error of itacion. bifocal lenses are also used. Bifocal lenses were devised by Benjamin Franklin, who was inconvenienced by constantly having to change his spectacles, for near and far vision. The early bifocals, consisting of two half lenses had a visible line of demarcation between the two pieces of gla . Later @ semi ar piece of gla , properly ground was cemented on the lower part of a full size lens. This method also left a visible line; in order to remedy this defect, machinery was developed to grind one piece of gla so that the upper and lower portions had different focal lengths