Watchmakers' eyeglasses are numbered according to their focal length in inches. A glass of short focus is stronger than one of long focus, and has to be used closer to the work. Thus with a 2-in. glass, the work is held 2in. from the glass, and so on. A 3i-in. or 1-in. glass is found the most convenient for ordinary work. 1 f spectacles are not used, order a 4-in. glass.