The quest for the human ocular accommodation mechanism.

We have known the accommodation phenomenon since 400 BC. Hypotheses about its mechanisms varied widely for two millennia. Early in the 17th century, when people became more aware of ophthalmic optics, Scheiner and Descartes were close to solving that accommodation worked by changes in the lens. Others rejected their idea, and people even denied the existence of accommodation because there was no clear proof. In the early 19th century, evidence accumulated for accommodation mechanisms studying bird, fish, insect, mammal and human eyes. On the discovery of muscle fibres in the ciliary body, attention shifted to its role in accommodation. Around 1850, came the proof that accommodation occurs by a change in the anterior lens curvature. Still for another 50 years, controversies remained about the exact changes in the lens and the precise accommodation mechanism. On looking back, this is not surprising because only late in the 20th century did it become clear that one cannot extrapolate from the multitude of accommodation mechanisms in the animal kingdom to human eyes.