John Marshall: the making of true spectaclesBMJ 1994; 309 doi: https://doi.org/10.1136/bmj.309.6970.1713 (Published 24 December 1994) Cite this as: BMJ 1994;309:1713
- D J Bryden, keeper London NW11 8AGa,
- D L Simms, retireda
- aDepartment of Science, Technology, and Working Life, National Museums of Scotland, Edinburgh EH1 2JF
In 1693 John Marshall of London devised a novel method of grinding batches of identical, good quality, lenses of a specified focal length. Its commendation by the Royal Society led to a trade war between Marshall and rivals in the Worshipful Company of Spectacle Makers. Despite initial opposition the method was rapidly adopted by London opticians and, though much modified, it forms the unrecognised basis of present day practice.
Spectacles were invented in the 13th century but their effectiveness was limited. The frame was perched on the nose and the customers selected the lenses that seemed to suit them best, a practice that has recently been reintroduced. The worst problem was that the lenses were poorly ground and made from poor quality glass despite the efforts of scientists and craftsmen to improve both material and technique.
There were several stages in producing lenses. Templates for shaping the grinding tools were made in pairs with an identical radius of curvature—one convex, the other concave. Pairs were turned to give a range of focal lengths. The templates and tools were normally made of iron and smoothed as far as possible. The processes of grinding and polishing were similar. The glass blank was cemented to the top of the convex tool. This was placed on a pole while the operator worked the concave tool in bold strokes by swinging the whole body, trying to cover the whole …