'Pan' one piece glass bifocal lens blanks
 Catalogue Number: 1745 'Pan' one piece glass bifocal lens blanks Category: Spectacles and lenses Time Period: 1900 to 1939 Description Of Item: Pair of uncut 'Pan' glass semi-finished one piece bifocal blanks 52 x 43 mm in original manufacturers boxes designated (A) Base 4.0 seg 32 add +4.0 and (B) Base 4.0 seg 32 mm add 3.75. Historical Significance: Benjamin Franklin is generally credited with the invention of bifocals in 1784. These were two separate half lenses fitted together in the spectacle frame. They remained in use for over 100 years. Cemented bifocals were invented in 1884 and fused bifocals were invented in 1890. They had a lens segment of higher refractive index inserted in a concavity ground in the main lens and fused in place. One piece bifocals were invented in 1906. These were a single lens but with two different surface curvatures so there was an edge between the distance and near part of the lens. How Acquired: Donated by John Kennedy, Kennedy Optical Supplies, Melbourne optical wholesaler Date Acquired: c 1975 Condition: Good Location: Archive office. East wall shelves unit 3. Drawer 4 |