SIR William Lower may well have been indulging in a post-prandial snifter or two when he wrote of what he had seen through his “Dutch trunke”. The moon’s surface, the English nobleman reported in 1609, “appears like a tart that my cooke made me last weeke; here a vaine of bright stuffe, and there of darke, and so confusedlie all over.”
His enthusiasm was part of a craze for astronomical observation that swept Europe in the early 17th century following the invention of the telescope. The first mention of a “device for seeing things far away as if they were nearby” comes in a patent filed in 1608 by the optician Hans Lipperhey of the Dutch city of Middelburg.
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But why Middelburg, and why 1608? It turns out that wine – or at least, the need for exquisite glassware to hold it – had more than a little to do with it. The same pattern has been repeated throughout the telescope’s history. Even today, we are looking at the universe through the bottom of a glass.
In the late 16th century, the Low Countries were a magnet for craftsmen from all over Europe. Thanks to its trade in spice from the East Indies, American silver and English wool, Antwerp in modern-day Belgium was Europe’s richest city. When Spanish forces invaded Antwerp in 1585 – part of a wearisome battle for hegemony known as the Eighty Years’ War – many of its leading merchants and artisans moved north to Middelburg.
Among them was Venetian glassmaker, Antonio Miotti. By 1606, he was running his own factory in Middelburg. Besides a lucrative line in coloured beads for trading with native North Americans, Miotti’s main output was “façon de Venise” wares, including drinking glasses, cruets, jugs and vases.
If you want to pick up superlative glassware, even today you can head for the Venetian island of Murano. Venice’s reputation stretches back 700 years. As the Mediterranean’s premier trading city, its secret was contacts in Egypt and Syria. These countries’ natural deposits of sodium carbonate, or soda ash, made Venetian glass the clearest, most sparkling in Europe. Thanks to Miotti and the twin forces of global trading and war, Middelburg now had it too.
For the invention of the telescope “it probably was advantageous that in 1608 Middelburg had a Venetian glass factory, making clear glass”, says Dutch historian of science Klaas van Berkel of the University of Groningen. But that wasn’t the whole story. Look at almost any early telescope – probably the earliest known example was by workers excavating a new railway tunnel in the Dutch city of Delft – and you’ll see a diaphragm in front of its focusing lens, a metal ring with a central hole that allows light to pass only through the middle. Even lenses made of the clearest glass were misshapen around the edges, defocusing light that passed through.
The diaphragm was probably Lipperhey’s Eureka moment, says Swiss optical engineer Rolf Willach. “For anyone not familiar with practical optics the results are astonishing,” he says. Willach’s own experiments looking through an old-school lens with a diaphragm at an optician’s chart bear that out. “The whole text becomes sharp and clear, nearly as good as seen with a modern optical instrument.”
But even with a diaphragm, the best Venetian glass was limited by physics. A lens focuses light of different wavelengths to different points, surrounding the final image with a slight halo of colour. The best way to cut down this “chromatic aberration” was to use a lens with a very long focal length. By 1673 in Danzig, now Gdansk in Poland, the brewer and astronomer Johannes Hevelius was making observations through a telescope .
A resolution was soon at hand. In late 17th-century England, glass was still pretty rubbish – after a few years, its surface “crizzled”, breaking up into a fine network of cracks. Rather than the scarce sodium carbonate, local glassmakers were using potassium carbonate, readily available from wood ash. Entrepreneur George Ravenscroft, under contract to the newly founded Worshipful Company of Glass Sellers, employed the glassmaker John Baptista da Costa – again an Italian – to try out some alternative chemistries.
One unusual ingredient Da Costa used was lead oxide, frowned upon on the continent as it turned glass yellow and destroyed the melting pot in which it was made. According to glass historian David Watts, the formula worked because furnaces in England burned coal, not wood, which meant glassmakers added potassium nitrate, or saltpetre, to oxidise dark specks of soot and prevent them blackening the glass. Saltpetre stopped lead oxide damaging the melting pot and also removed the discolouration.
So lead crystal glass was born, patented by Ravenscroft in 1674. Soft and heavy, it could be cut into faceted glasses and decanters that scintillated with different hues, dispersing the colours of the spectrum in a different way from the traditional “crown glass”.
An Essex barrister, Chester Moor Hall, put two and two together. He commissioned a jobbing lens maker to combine a pair of lenses, one of crown glass and the other of lead glass. Lo and behold, the colour dispersion of the two cancelled out, focusing light to a sharp, pure white.
Moor Hall never built a telescope, but the lens maker got talking to a scientific instrument-maker, John Dollond, who patented the idea and made a fortune. Astronomers could now use short, precise telescopes to measure positions in the sky and calculate the distances to the stars.
“Astronomers were inspired by tableware coated in silver nitrate and grape juice”
Glass is heavy, however, and by the end of the 19th century, telescope lenses could not get any larger. The , near Chicago, was a metre across. Any bigger, and it would sag under its own weight, distorting the image.
At the time, astronomers were enthralled by “faint fuzzies” in the sky – what we now know are distant galaxies. To probe these dim objects, they needed to grab as much light from them as they could. They were going to need a bigger telescope.
This time, the future of the telescope had actually emerged some decades before, at the Great Exhibition of 1851 in London – again in the tableware section, where visitors were dazzled by glass goblets and candlesticks shining as brilliantly as pure silver. A pair of London glassmakers, Hale Thomson and Edward Varnish, had taken out a patent for chemically coating silver – from a mixture of silver nitrate and grape juice – onto the inside of glass vessels. Named mercury glass for its shiny fluid appearance, the fad quickly spread to Germany and the US.
Around this time the German astronomer Carl von Steinheil was trying to improve reflecting telescopes, which use a concave mirror to reflect light and so focus it. Unlike a lens, a mirror can be supported from behind, so there’s no problem with sagging. The basic idea dated back to Isaac Newton in the 1660s, but speculum, an alloy of copper and tin used by astronomers, was weighty and needed frequent polishing. Worse, it typically reflected only two-thirds of the light falling on it.
Mercury glass reflected over 90 per cent. In 1858, Steinheil reported to the Royal Astronomical Society in London “a result that appears likely to lead to a total modification in the construction of astronomical instruments. Do not be alarmed…” These reflectors might, Steinheil said, be built “at a moderate outlay, which is a point that should not be by any means overlooked”.
Independently, Léon Foucault in France – he of pendulum fame – had the same inspiration. Soon the silver-on-glass mirror became standard for building ever bigger telescopes to peer farther into space. The at the Mount Wilson Observatory in California, used by Edwin Hubble to observe the motions of distant galaxies and discover that the universe was expanding, is one example. Other metals have since been used to “silver” astronomical mirrors; in particular, aluminium. When the , successor to the Hubble Telescope, is launched in 2018, astronomers will view the deep recesses of the universe reflected in a thin layer of pure gold.
Interest in mercury glass for tableware waned long ago. But like many innovations from the salons of the chattering classes, its influence on astronomy has been long-lasting. Silver’s superior ability to reflect infrared light means that the original mercury glass is still used, for example, in the 8-metre mirrors of the giant Gemini telescopes in Hawaii and Chile, and for the planet-hunting in orbit around the Earth. Time to raise a glass to the enduring art – and science – of the glassmaker.
(Images: Gemini Observatory, Sarah Lee/Eyevine, Stefano G. Pavesi/Contrasto/Eyevine)
This article appeared in print under the headline “The universe, decanted”
