Early Lens Making

By Holly Tucker

As part of my day job, I write. I research. I write some more.

There are moments when it's hard to cut out hard-earned words. This is one of them. But I also know that--at least for this chapter that I just finished buttoning up--readers' eyes could glaze over in all the details about early telescopes and the challenges of lens making. It had to go.

So here I am, asking your indulgence as I put a least some of that writing to use in a post...

In the seventeenth century, the challenge for astronomers was to be found in the lenses. Lenses were extraordinarily difficult to make, and the quality was inconsistent at best.

The best available glass for telescope lenses was concocted from a mixture of sand and soda, borax was then added along with a bit of lime and manganese oxide. These last two ingredients tempered the yellow and greenish hue in the glass, not entirely but at least noticeably. All of the materials were left to boil in gigantic melting pots until the various components fused into a unified liquid. Sweating and covered in soot, the glassmaker then began the delicate and dangerous process of scooping the molten liquid onto marble slabs and rolled thin layers flat. The glass would be built up progressively, in layers, until just the right thickness had been achieved. Anything could go wrong in the process--and it usually did. Bubbles, particulates, and fissures specked the glass. And even the most faultless piece of glass was never perfectly clear. "The good colors of glass," wrote the astronomer and instrument-maker Cherubin, "are approaching wine-colored, or blue, or green, even black, but always transparent. Green or the color of sea water is most common."

Once the rough shape of the lens was traced out and cut with a diamond, the grinding process began. [Imagine here several paragraphs of minutiae on lens griding--also deleted!] It could take twenty, thirty, and sometimes more tries to get one lens that was of sufficiently high enough quality for use in a telescope.

Now before you think that I've gone the way of history of engineering, know that much of this great information is indebted to Maurice Daumas'
Scientific Instiruments of the 17th and 18th Centuries and Their Makers. Another excellent book is J.A. Bennett's The Divided Circle: A History of Instruments for Astronomy, Navigation and Surveying. Or, the visually stunning Scientific Instruments: 1500-1900, an Introduction by Gerard L'E Turner.

Image courtesy of the Wellcome Library for the History of Medicine

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  1. Say, Holly - I suppose you know that this year is the 400th anniversary of Galileo's telescope? Timely post you have here!

  2. The thing that strikes me is how much we take clear glass and even reliably good lenses for granted these days.

    Great information. Thanks for sharing it. And I know what you mean about having to cut great details and facts. It's always heartbreaking. But glad it made it to your blog.

  3. Fascinating post! I never thought about how hard it was to make lenses...of course it would be in the 17th century!


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