Donders, Franciscus Cornelius (1818-1889)

Dutch oculist and ophthalmologist who made important advances in the prescribing and fitting of spectacles. He wrote a classic textbook on the optics of the eye and devised sets of letters of different sizes for testing a patient's visual acuity, diagnosing sight defects, and prescribing spectacles.

In his book The Anomalies of Refraction and Accommodation 1864, Donders separated the errors of refraction (the bending of light) from those of accommodation (the change in the shape of the lens of the eye to maintain focus). He describes the errors in refraction as constant, resulting from light being focused at a constant point either behind or in front of the retina, depending on the shape of the eye. Errors in accommodation, Donders discovered, result from defects in the eye's focusing machinery, such as weak ciliary muscles (the muscles that change the shape of the lens) or a hardening of the lenses.