Christiaan Huygens’ telescope lenses inform us that he was short-sighted
A world-renowned 17th-century astronomer credited with discovering Saturn’s moon Titan may have needed glasses, according to a recent article in the Royal Society Journal of the History of Science.
At a time when telescope technology was less than a century old and evolving rapidly through trial and error iterations, Christiaan Huygens was known for producing lenses of unparalleled quality. However, the telescopes he built with these lenses consistently underperformed. The cause, according to AIP researcher Alex Pietrow, may have been myopia, or nearsightedness, which was common in the Huygens family, although his case must have been mild enough not to be noticed.
“Because Huygens didn’t need glasses in everyday life like his father did,” says Pietrow, “he probably didn’t think about it when making telescopes. So he subconsciously incorporated that eye defect into his designs.”
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How do we know Huygens was myopic? We’re not sure: diagnosing diseases in historical figures is a delicate matter, and claiming to be able to do so with certainty is quite a malpractice in two professions – medicine and history. So Pietrow rightly safeguards his recipe with the appropriate degree of uncertainty. But he makes strong arguments by examining the mathematical formulas that Huygens developed through trial and error to understand the physics of optics and comparing them to modern formulas. The telescopes that Huygens built from his formulas had lower resolving power compared to some of his contemporaries and could be explained if he unconsciously took into account his own poor eyesight.
Pietrow offers a few other possible explanations for the poor performance of Huygens’ telescopes, some more plausible than others. For example, if the Huygens telescope lenses suffered from polishing defects, this would result in a similar reduction in resolving power. Without performing a test called a wavefront measurement on one of Huygens’ surviving lenses, this cannot be completely ruled out, but Pietrow believes this is an unlikely explanation given that Huygens’ lenses were by all appearances exceptional. For example, in 1846 Frederik Kaiser, former director of the Leiden Observatory, described Huygens’ lenses as “flawless”. Huygens’ contemporaries would have found it difficult to improve his techniques, nor would he have been very interested in sharing his methods with them.
“In Huygens’ time, lens making was a very mysterious art,” Pietrow told Universe Today. “Different lens manufacturers competed to be the best while keeping their techniques secret. That means polishing the lenses, sourcing the glass, etc. were trade secrets that nobody talked about.”
Christiaan Huygens, by Bernard Vaillant (1686). Photo credit: Wikimedia Commons.
A third, and perhaps more plausible, possibility is that Huygens intentionally built its telescope lenses with a shorter focus than necessary—a “suboptimal” design choice—but one that offered a distinct advantage. This made it possible to shorten the overall length of the telescope considerably.
This was an important advantage because the best telescopes of the time reached unwieldy lengths, some reaching over 50 meters end-to-end.
The reason for their excessive length was the need to correct what is known as chromatic aberration.
When light passes through a lens that is thicker in the center and thinner at the edges, the light bends, but different wavelengths bend at different angles. Short wavelengths, such as those that produce the color blue, converge faster than longer, reddish wavelengths, leaving an annoying color cast that surrounds whatever object you view through the telescope.
A slight chromatic aberration can be seen at the edge of the moon. Photo credit: Wikimedia Commons
The eventual solution to chromatic aberration was to use mirrors instead of glass – this is how Isaac Newton built his telescopes – a technique that circumvented the problem entirely. But Huygens applied a different solution, and that was lengthening his telescopes, which also significantly reduced the effects of chromatic aberration.
The downside was that it made his telescopes impractically large.
In fact, Huygens’ largest telescopes were so difficult to use that he made most of his important discoveries with smaller telescopes. The telescope with which Huygens discovered Titan was only 3.6 m (12 ft) long, and using either the same 3.6 m telescope or a new 7 m (23 ft) telescope, he was the first who realized that Saturn’s odd appearance was due to a series of rings.
Huygens with one of his “tubeless” air telescopes. Photo credit: Wikimedia Commons.
In the fight against chromatic aberration, Huygens may have intentionally reduced the performance of his lenses to reduce the overall length of his instruments.
Nonetheless, Pietrow considers it unlikely that Huygens intentionally introduced such an error into his instruments. “We know,” he writes, “that Huygens tried to build the optimal telescope.” For Pietrow, visual impairment remains the most likely remaining alternative.
Whether or not prescription glasses might have helped Huygens see the universe a little more clearly, the man undoubtedly helped the rest of humanity see it more clearly. He left a scientific legacy that astronomers and mathematicians still build on, and a bit of myopia, if any, didn’t get in his way.
Alex Pietrow: “Did Christiaan Huygens need glasses? An Examination of Huygens’ Telescope Equations and Tables.” Royal Society Journal of the History of Science.
“Eyewear prescription for Christiaan Huygens after 330 years.” AIP.
Featured Image: A collection of lenses by Christiaan Huygens with his portrait in the background. Photo credit: Rijksmuseum Boerhaave, Leiden.
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