How are Bifocals made?

Bifocals are spectacles in which each lens is made in such a way that the upper and lower halves have different focal lengths. This helps the wearer overcome the inconvenience of needing two separate pairs of spectacles, one for reading and the other for seeing distant objects. In the usual type of bifocals called “split” bifocals, two separate lenses are fitted in the same frame. The upper segments are for distant vision and lower one for near vision. The “fused” invisible bifocals are made by combining a lens of crown glass with a segment of flint glass. First a depression is made in the lens of crown glass and then a button of flint glass, similarly polished and ground, is laid upon it, the two are then clipped together and heated in a furnace to 600°C until they fuse together. The third “solid” types of bifocals are one-piece bifocals. They are made of one piece of glass; two distinct curvatures are ground upon one spherical surface. These are free from optical defects.