Christiaan Huygens’ Telescope Lenses Tell Us He Was Nearsighted

A world-famous 17th-century astronomer credited with discovering Saturn’s moon Titan may have needed glasses, according to a recent paper in the Royal Society Journal of the History of Science.

In an era 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 those lenses consistently underperformed. The cause, AIP researcher Alex Pietrow suggests, may have been myopia, or nearsightedness, which was a common condition in the Huygens family, though his case must have been mild enough not to notice.

“Because Huygens did not need eyeglasses in daily life like his father,” says Pietrow, “he probably did not think about it when making telescopes. So he unconsciously included this eye defect into his designs.”

How do we know Huygens was nearsighted? We don’t for sure: diagnosing medical conditions in historical figures is tricky business, and claiming to do so with certainty is pretty much malpractice in two professions – medicine and history. So Pietrow rightfully hedges his prescription with the appropriate degree of uncertainty. But he makes a strong case by examining the mathematical formulas Huygens developed via trial-and-error to understand the physics of optics, and comparing them to modern formulas. The telescopes Huygens built using his formulas ended up with inferior resolving power compared to some of his contemporaries, and might be explained if he was unconsciously accounting for his own impaired eyesight.

Pietrow offers a few other possible explanations for the underperformance of Huygens’ telescopes, some more plausible than others. For example, if Huygens’ telescope lenses suffered from polishing errors, it would produce a similar reduction in resolving power. Without doing a test called a wavefront measurement on one of Huygens’ surviving lenses, this can’t altogether be ruled out, but Pietrow believes this is an unlikely explanation, because by all accounts Huygens’ lenses were exceptional. For example, in 1846 Frederik Kaiser, former director of the Leiden Observatory, described Huygens’ lenses as ‘pristine’. Huygens’ contemporaries would have been hard-pressed to improve on his techniques, and he wouldn’t have been keen to share his methods with them either.

“In the time of Huygens, lens making was a very secretive art,” Pietrow told Universe Today. “Different lens makers competed to be the best, while keeping their techniques secret. This means that polishing the lenses, sourcing the glass, etc. were all trade secrets that no-one talked about.”

A third possibility, and perhaps a more plausible one, is that Huygens purposefully built his telescope lenses with a shorter focus than necessary – a ‘sub-optimal’ design choice – but one which offered one key advantage. It allowed the total telescope length to be significantly shorter.

This was an important advantage, because the best telescopes of the period were reaching unwieldy lengths, some reaching over 50 meters from end to end.

The reason for their excessive length was the need to correct for what is known as chromatic aberration.

When light passes through a lens that is thicker in the middle and thinner on the edges, the light is bent, but different wavelengths are bent at different angles. Short wavelengths, like those that produce the color blue, will converge more quickly than longer reddish wavelengths, leaving an annoying colorful tinge surrounding whatever object you’re looking at through the telescope.

The eventual solution to chromatic aberration was to use mirrors rather than glass – this is how Isaac Newton built his telescopes – a technique that bypassed the problem altogether. But Huygens employed a different solution, and that was to increase the length of 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 using smaller ones. The telescope Huygens used to discover Titan was only 3.6m (12ft) long, and using either the same 3.6m telescope or a new 7m (23ft) one, he was the first to discern that Saturn’s odd appearance was due to a series of rings.

In the battle against chromatic aberration, Huygens may have purposefully diminished his lenses’ capability in order to reduce the overall length of his instruments.

Still, Pietrow thinks that it is unlikely that Huygens’ would intentionally introduce such an error into his instruments. “We know,” he writes, “that Huygens was trying to make the optimal telescope.” For Pietrow, that leaves vision impairment as the most likely remaining alternative.

Regardless of whether prescription glasses might have helped Huygens see the Universe a little clearer or not, the man undoubtedly helped the rest of humanity see it more clearly. He left behind a scientific legacy upon which astronomers and mathematicians are still building, and a little bit of myopia, if it existed, didn’t get in the way.

Learn More:

Alex Pietrow, “Did Christiaan Huygens need glasses? A study of Huygens’ telescope equations and tables.” Royal Society Journal of the History of Science.

“Eyeglass prescription for Christiaan Huygens after 330 years.” AIP.

Featured Image: A collection of lenses by Christiaan Huygens with his portrait in the background. Credit: Rijksmuseum Boerhaave, Leiden.