Bifocal

(bi-fof-kal) \_bi, two; focus, a point]. Having a double focus. Applied to a system of lenses or spectacle-gla es with two foci, chiefly used for the correction of presbyopia, wThen there is at the same time an error of refraction for distant vision. The distance lens is above that for near work. These are sometimes called Pantoscopic lenses, and also Franklin spectacles, because the device was first made by Benjamin Franklin. They are also called Cement lenses, because now made by cementing the lower segment to the distance lens. 146