What does 'achromatic' mean? Refractions on the construction of early achromatic telescope lenses

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• Michael Korey (Mathematisch-Physikalischer Salon of the Dresden State Art Collections)
• Thursday 14 November 2024, 15:30-17:00
• Hopkinson Lecture Theatre, New Museums Site.

If you have a question about this talk, please contact Dr. Rosanna Dent.

Anita McConnell Lecture

Drawing on the work of telescope collector and optical engineer Rolf Willach, this richly illustrated presentation looks at central, yet previously unexamined aspects of the transition from single-lens telescope objectives to doublet and triplet lenses made by John and Peter Dollond.

Whereas the thorough, detailed investigation by Brian Gee (completed by Anita McConnell and Alison Morrison-Low) of the Dollond patent dispute focused on archival and printed sources about the innovation, studies by Willach and Duane Jaecks have concentrated on the subtle optical physics that distinguishes the new compound lenses from their predecessors. Still other work by Roger Ceragioli, and more recently by Huib Zuidervaart and Tiemen Cocquyt, has brought together archival and optical research, to generate even deeper insights into this episode.

At the heart of the transition is the problem of chromatic aberration, which can be stated rather simply: when white light encounters a lens, different colours come to focus at different points, causing images with coloured fringes and annoying prismatic bands. By the 1650s, the problem was widely lamented, and a solution was desired. In fact, many solutions were developed (both in theory and practice), were used, and were even advertised.

But what was the actual problem? And what did it mean to solve the problem? That is, what did it mean to be achromatic? To our surprise, the answers in the 1750s and following decades were surprisingly vague; rather curiously, recent scholarship has not discussed this at length either. And looking through old telescopes, as the lecturer has done for decades with his collaborator Marvin Bolt, raises serious questions about the accuracy of traditional tales. We need to broaden our analyses, especially to avoid anachromisms from colouring our historiography.

Willach’s latest research explores, for the first time ever, a plausible method by which Dollond could have produced achromatic lenses using resources available to him, including simple algebra, well-known experimental methods, and (most significantly) carefully made prisms. Our efforts to clarify his analysis also highlight the need to address a seemingly simple question: what does ‘achromatic’ mean?

This talk is part of the Departmental Seminars in History and Philosophy of Science series.

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