On a cloudy fall morning in 1880, Willy Kuhne, a distinguished professor of physiology at the University of Heidelberg, waited impatiently for 31-year-old Erhard Reif to die. Reif had been found guilty of the reprehensible act of drowning his own children in the Rhine, and condemned to die by guillotine. Kuhne’s eagerness for Reif’s death, however, had nothing to do with his desire to see justice served. Instead, his impatience was mostly selfish—he had been promised the dead man’s eyes, and he planned to use them to quell a bit of scientific curiosity that had been needling him for years.

For the several years prior, Kuhne had been obsessed with eyes, and especially with the mechanism underlying the eye’s ability to create an image of the outside world. As part of this obsession, Kuhne wanted to determine once and for all the veracity of a popular belief that the human eye stores away an image of the last scene it observed before death—and that this image could then be retrieved from the retina of the deceased. Kuhne had given these images a name: optograms. He had seen evidence of them in frogs and rabbits, but had yet to verify their existence in people.

Optograms had become something of an urban legend by the time Kuhne started experimenting with them. Like most urban legends, it’s difficult to determine where this one began, but one of the earliest accounts of it can be found in an anonymous article published in London in 1857. The article claimed that an oculist in Chicago had successfully retrieved an image from the eye of a murdered man. According to the story, although the image had deteriorated in the process of separating the eye from the brain, one could still make out in it the figure of a man wearing a light coat. The reader was left to wonder whether or not the man depicted was, in fact, the murderer—and whether further refinements to the procedure could lead to a foolproof method of identifying killers by examining the eyes of their victims.

Optograms remained an intrigue in the latter half of the 19th century, but they became especially interesting to Kuhne when physiologist Franz Boll discovered a biochemical mechanism that made them plausible. Boll identified a pigmented molecule (later named rhodopsin by Kuhne) in the rod cells of the retina that was transformed from a reddish-purple color to pale and colorless upon exposure to light. At the time, much of the biology underlying visual perception was still a mystery, but we now know that the absorption of light by rhodopsin is the first step in the visual process in rod cells. It also results in something known as “bleaching,” where a change in the configuration of rhodopsin causes it to stop absorbing light until more of the original rhodopsin molecule can be produced.

In studying this effect, Boll found that the bleaching of rhodopsin could produce crude images of the environment on the retina itself. He demonstrated as much with a frog. He put the animal into a dark room, cracked the windows’ shutters just enough to allow a sliver of light in, and let the frog’s eyes focus on this thin stream of light for about ten minutes. Afterwards, Boll found an analogous streak of bleached rhodopsin running along the frog’s retina.