The German physiologist Wilhelm Friedrich Kühne (1837–1900) coined the term rhodopsin for the pigment that is responsible for both the formation of the photoreceptor cells and the first events in the perception of light.

In 1878 he did a fascinating experiment. He covered a rabbit’s head to allow the rhodopsin to accumulate in the rods. Then it was uncovered and positioned toward a barred window. After a three-minute exposure, the animal was decapitated and the eyeball removed. He then ‘fixed’ the retina with a solution to prevent it from being resynthesized. The next day, Kühne was able to see a picture of the window and bars printed upon the retina:

Retinal Photograph, drawn by Willy Kühne, 1878