Was Christiaan Huygens nearsighted?

In 1655, using a telescope he designed, Christiaan Huygens became the first person to identify Saturn’s rings and one of its moons. In that era, the still-nascent understanding of optics was driven by trial and error: Researchers would find and optimize the focal lengths and other properties by eye and create tables of those values. The relationship between, say, the lens diameter and focal length could then be approximated from the tables. That process is what Huygens followed when he constructed his telescopes.

In some cases, Huygens managed to derive accurate relationships—for example, he correctly identified the magnification as the ratio of the focal lengths of the objective lens and the eyepiece, f_o/f_e. But many of the equations and tables are notably off from what one would expect on the basis of modern geometric and diffractive optics. In a recent study, Alexander Pietrow of Stockholm University in Sweden proposed that the discrepancy may have arisen not from limitations in Huygens’s skill or understanding but rather from his poor eyesight.

Pietrow found that Huygens’s combination of objective and eyepiece lenses overmagnifies by a factor of 3.5. On the basis of modern optics and typical human eyesight, the magnification from a telescope is related to the diameter of the objective lens by the equation M = 4.33D_o. Although Huygens correctly identified a linear relationship, he empirically derived the equations M = 13.89D_o and M = 15.29D_o in two of his works.

Huygens’s equations could be reconciled with modern optics if his visual acuity had been 20/70. That level of nearsightedness wouldn’t require glasses, and Huygens’s father was known to have severe myopia, which has genetic risk factors. Pietrow suggests that Huygens may have unknowingly corrected for his eyesight in the design of his telescopes.

There are other potential explanations for the suboptimal design of Huygens’s telescopes. For example, if the lens surfaces are too rough or misshapen, then the limits on the performance aren’t based on the diffraction limit. The lenses had a reputation for high quality at the time, but future work could test whether they do indeed produce a diffraction-limited spot, as was shown for those made by the 17th-century optician Giuseppe Campani. (A. G. M. Pietrow, Notes Rec., 2023, doi:10.1098/rsnr.2022.0054 .)