Helmholtz's ophthalmoscope

The pupil looks black, and for centuries no one could see what lay behind it. In 1810 Bénédict Prévost explained that the eye glows only when light enters it from outside. Jan Purkinje saw the back of a dog's eye, and then a human eye, by reflecting candlelight from his spectacles, but his 1825 account in Latin went unnoticed. Charles Babbage built an instrument in 1847, but it gave no image when he showed it to the ophthalmologist Thomas Wharton Jones, and he gave up.

Hermann von Helmholtz, a 29-year-old professor of physiology, wanted to show his students why the pupil sometimes looked black and sometimes glowed. The American Academy of Ophthalmology's museum dates his instrument to 6 December 1850 and says he made it from cardboard, glue and microscope glass while preparing a demonstration of the conservation of energy. He described it in 1851 and called it an Augenspiegel, or eye mirror; the name ophthalmoscope came into common use in 1854.

Helmholtz set out what such an instrument needs: a light source, a way to send the light into the eye along the observer's line of sight, and optics to sharpen the blurred image of the fundus. His answer was a stack of thin glass cover plates set at an angle. They reflected the light of a candle into the patient's eye while the observer looked through them at the retina, and he tried different numbers of plates, noting that three or more partly polarized the light.

Others soon changed the design. Theodor Ruete of Leipzig built the first instrument for indirect ophthalmoscopy in 1852, and mirrors with a hole in the center replaced the glass plates. By the time Helmholtz died in 1894 a great many models existed; an exhibition for the instrument's 50th anniversary showed 140 designs, and in 1913 Edward Landolt counted 200.