A Summary of January’s main meeting

Bob Marriott (BAA Instruments’ Section) talked about the ‘Silver on glass revolution’.

The 1850s witnessed a massive revolution in science and astronomy.It was long known that reflector telescopes could give good results and that experimentation found that a copper and tin combination (two thirds copper, one third tin) gave reasonably good reflectivity, especially when a little arsenic was added. Unfortunately the arsenic made the surface tarnish very easily, so the hunt was on to see if it was possible to deposit a metal film on glass. (When John Herschel was working in Cape Town in the 1830s he used three 18” mirrors in rotation in his telescope. They had to be repolished and refigured every two to three weeks.)

Both Justus Liebig, an industrial chemist (also of Oxo and Marmite fame), and Michael Faraday experimented with using alkalides to deposit metals on glass. Faraday was only about 18 at that point.
John Stenhouse (who invented the carbon filter which became the basis for the gas mask) did lots of experimental work, adding various oils into the mix.

By the 1850s a silver nitrate preparation with a glucose reducing agent (look it up!) was able to give a good result, but unfortunately the glass itself was still not optically good enough for use in reflectors, even the best Venetian glass.

The 1850s were very busy as regards events: the Hyde Park Great Exhibition (1851), Cholera epidemic (1854), and Crimean war (end 1850s onwards). James Clerk Maxwell was playing with photography and by 1861 had managed to take the first colour photograph. In the meantime Warren de la Rue wrote on the progress of silvering and Webb’s ‘Celestial objects for Common Telescopes’ came out in 1859, the same year as the massive Carrington event (solar eruption).

The art of silvering glass was finally perfected by Leon Foucault in 1857, but speculum metal mirrors continued to be made.

What pervaded Ian’s high-speed talk was the enthusiasm that the Victorian astronomers and scientists had. Many of the names are familiar to us; do look them up: William Lascelles, James Nasmyth, William Huggins, Henry With, George Calver. And they were almost all amateurs.

Gwyneth Hueter.

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