|
Bifocals were invented by Benjamin Franklin. The original design consisted of two glass lenses, one for distance vision and one for near, cut to the same shape to fit a particular spectacle frame. They were then cut exactly in half and the top half of the distance put in the top of the frame and the bottom half of the near put in the bottom of the frame, the process repeated for the other eye. A simple design and in many ways superior to the sophisticated bifocal lenses of today in that it has the scope to accomodate unusual and difficult prescriptions. The big disadvantage is the appearance of the lens. It would not be easy to persuade a patient today to wear such a lens, certainly not in a modern designer frame!
Following this ground breaking developement in lens technology we had the solid invisible bifocal, glass of course and not particularly invisible, but a considerable improvement , from the aesthetic point of view than Mr. Franklin’s product. The solid bifocal basically is one piece of glass, ground with a tool to produce a high powered surface in the centre and another tool to produce a lower powered surface around it. If one can imagine a saucer with, instead of a depression in the middle, a spherical “mound” , then that is basically the first stage of a solid bifocal lens. This first stage can then be cut into two parts to produce two lenses with a “half moon” segment at the bottom. The lens can then be surfaced, (ground in layman’s terms) on the other side to complete the required prescription, and cut to the required shape and size.
Solid glass bifocals are seldom used today and even the same designs in plastic are becoming rarer. Naturally, Franklin, sometimes known as “split Franklin” or just “split” have virtually disappeared. There are however, still one or two aged opticians who (with difficulty and plenty of advanced notice ) can give one the benefit of their knowledge of the subject.
|
|
