'Austral' fused bifocal lens blank

Catalogue Number: 1746
'Austral' fused bifocal lens blank
Category: Spectacles and lenses
Sub-Category: Spectacle lenses
Time Period: 1900 to 1939
Place Of Publication/Manufacture: Australia
Publisher/Manufacturer: Austral
Description Of Item: Fused bifocal semi-finished lens blank, 55 mm diameter, tinted, crown glass, + 6 D base curve. Flint glass segment button, 27 mm diameter, marked on front 'Ton B + 6.00' and '- 3.00' back surface. In original blue and white box marked ''AUSTRAL' FUSED BIFOCALS Distance Melb Addition D 22' on one side and 'Made in AUSTRALIA by Australians for Australians All lenses Guaranteed' and on the end of the box 'No .... TORIC +6.00 Ton B'
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: Very good
Location: Archive office. East wall shelves unit 3. Drawer 4

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