Optical surfaces are polished by abrasion. "Polishing" was initially by hand; later, the optical lathe was developed, which, in its basic structure, is still used in lens-making today. The so-called "air lathe" [tornio in aria] used in Murano and Venice in the early 17th C. was activated by a flywheel rotated by the craftsman with a handle. Around the flywheel was placed a belt or rope that transmitted the motion to the horizontal lathe axle overhead. A mud-soaked rag or resinous material was fastened to a round plate at the end of the axle. The craftsman brought the glass up to the plate and polished the surface by pressing it down. The Florentine craftsman Ippolito Francini is credited for turning the "air lathe" into the present-day optical lathe, with a vertical axis. This modification gives the operator far greater control of the glass during the process and yields smoother, more regularly shaped surfaces. Other innovative lathes were devised in the second half of the 17th C.: a lathe by Campani, in Rome, with a horizontal axle and a long pressing rod that moves the glass and keeps it at the desired curvature; a lathe described by Chérubin d'Orleans, similar to Campani's, but with a vertical axle; and Hooke's lathe, with a vertical axle and an inclined, revolving rod.
Andrea Frati, Florence, second half 18th cent.