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The color purple: milestones in photochemistry

Earlier this year, the Illinois Eye and Ear Infirmary, a venerable institution that was later incorporated into the University of Illinois at Chicago College of Medicine, celebrated its 150th anniversary. Asked to deliver a lecture to the distinguished group of physicians and scientists that had assembled to celebrate this historic event, I was struck by the fact that in 1858, when the Infirmary opened its doors to the public, absolutely nothing was known about any aspect of the visual process. There were, to be sure, significant advances in ocular surgery, a growing number of medical treatments for eye disease, and vastly improved recognition of retinal disorders due in no small measure to the arrival two years earlier of Helmholtz’s ophthalmoscope. But knowledge of how the visual signal was generated had not progressed much further than that shown in Descartes’ fanciful cartoon (Fig. 1). The situation changed dramatically some twenty years later, when Franz Boll, a German physiologist working in Rome, removed the retina from the eye of a dark-adapted frog, and reported the color changes he observed as the tissue was exposed to light. Boll had witnessed the light-