Sources on franklin bifocals

[redfox11]

I am currently working on my thesis about franklin bifocals, but there almost no sources of comprehensive informations about when and how they are used in my native language, even though they were pretty common few decades ago. Google searches also yield mostly biographies of Ben Franklin and no usable sources

I stumbled across this forum and saw that some users still make them, so could you recommend books, articles, research papers ot anything really to help me?

[Lensman11]

Try searching under Executive bifocals as that was the branded name. Also search on the history of American Optical as they invented the one piece Executive lens. The Franklin was actually two pieces. Other companies that made the product were Shuron Optical it was named Kurova M and Univis Optical. In cr 39 the lenses were made by American optical Sola optical.