Ophthalmoscope, steel, brass and glass, by Collin and Co. of Paris, late 19th century
Herman von Helmholtz (1821-1894) invented the ophthalmoscope in 1851. It is a simple, highly effective instrument used to view the interior of the eye. Helmholtz reasoned an observer placing his eye in the path of light reflected from the back of a subject’s eye (through the pupil) could see the subject’s retina. Helmholtz used glass plates angled at 45 degrees to reflect sunlight into the eye. In this example, a ratchet mechanism rotates two wheels of lenses in front of a perforated mirror.