Round Safety Glasses
Safety glasses are usually made with shatter-resistant plastic lenses to protect the eye from flying debris. Although safety lenses may be constructed from a variety of materials of various impact resistance, certain standards suggest that they maintain a minimum 1 millimeter thickness at the thinnest point, regardless of material. Safety glasses can vary in the level of protection they provide.
Government Printing Office The Official Gazette of the United States Patent Office (Washington, DC: Government Printing Office, 1895)
Courtesy the private collection of Roy Winkelman