Optography’ results from the bleaching of the rhodopsin in those areas of the retina that have been directly affected by light. Rhodopsin is a light sensitive chemical substance by which the rods of the retina can distinguish in twilight between bright and dark. Bright spots in an optogram correspond to the area where the rhodopsin has been bleached by light. Dark spots designate the sector in which the rhodopsin is still intact. Unlike photography where the film is produced as a negative, optography generates a positive. A bright object appears bright, whereas darker motifs result darker because of the lower bleaching effect of dark colours.
About 130 years ago, the physiologist Willy Kuhne (1837-1900) from the University of Heidelberg discovered this phenomenon by accident. On the retina of a frog, he was able to detect the image of a gas flame the frog had been staring at for a while before it was killed in the laboratory. To confirm his observation, Kuhne conducted subsequent experiments with rabbits. In a dark room, he placed the animals in front of a bright window for a short amount of time, killed them and removed – still in the dark – their eye bulb. The image of the window appeared clearly on the isolated retina as a bright quadrangular spot. Kuhne called this image ‘optogram’.
By the middle of the 20th century other scholars were also able to obtain optograms on animal retinas. The possibility to uncover previously seen objects on the retina of a creature especially inspired the imagination of criminologists. In the mid-1970s they contacted the ophthalmic hospital of the University of Heidelberg requesting “whether and to what extent it would be possible to detect objects or men on the retina of a murdered person, what this person had seen immediately before being killed.” We considered this to be a very interesting question and revived Kuhne’s research to clarify the conditions on which an optogramm could be obtained.
Like Kuhne, we also used rabbits for our experiments. We anaesthetised the animals and located them in front of a screen on which patterns rich in contrast were projected. After a certain amount of time we killed the animals in the dark, only using illumination available in a photographic darkroom. We quickly removed their eye bulbs and detached the anterior part of the eye and the vitreous.
The rear part of the bulb with the retina was put into a 14% potassic alum solution for 24 hours. After that we isolated the retina, tightened it on a ball that had the same dimensions as the eye bulb, and let it dry in the dark. The original optograms are not very light-resistant. Therefore we took photographs of the results that show the patterns the rabbits had been looking at. For this experiment we also used a portrait of Salvador Dali as pattern that I had drawn with a black 1,5mm pencil on white paper. In this case we took a coloured photograph of the optogram.
The criminologists, however, had to abandon their hope of seeing the image of a murderer on the retina of his or her victim. Although this remains a theoretical possibility, it is impossible to obtain an optogram that would be usable for forensic purposes. The creation of a ‘readable’ optogram depends on a multitude of prerequisites that can be provided only in a laboratory. And how many murderers oblige their prosecutors by working under laboratory conditions?