Focusing by shape change in the lens of the eye: a commentary on Young (1801) 'On the mechanism of the eye'.

In his Bakerian Lecture paper of 1801, Thomas Young provided the best account up to that time of the eye's optical system, including refraction by the cornea and the surfaces of the lens. He built a device, an optometer, for determining the eye's state of focus, making it possible to prescribe appropriate correction lenses. His main contribution, however, was to show that accommodation, the eye's focusing mechanism, was not the result of changes to the curvature of the cornea, nor to the length of the eye, but was due entirely to changes in the shape of the lens, which he described with impressive accuracy. He was wrong, however, in believing that the reason the lens bulges when focusing on near objects was because it behaved as a contracting muscle. Half a century later, Helmholtz showed that the lens bulges not by its own contraction, but when it is relaxed as a result of contraction of newly discovered circular muscles in the ciliary body. This commentary was written to celebrate the 350th anniversary of the journal Philosophical Transactions of the Royal Society.