The ophthalmoscope is a simple, highly effective instrument used to view the interior of the eye. It was invented in 1851 by Herman von Helmholtz (1821-1894). 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. This example was made in England by instrument maker Ferrier. A ratchet mechanism rotates two wheels of lenses in front of a perforated mirror.