Wald and his teacher Hecht contributed to a tradition that goes back to the 1870’s. It is worthwhile to both address the history of the scientific understanding of vision that preceded these two as well as review the basics of visual physiology, as we understand it today.
All of the scientific work explored in this exposition falls within what is sometimes called “the duplicity theory of vision.” This is a research tradition in ocular physiology that holds that light sensitivity is processed by more than one substance in the eye. That is to say that the eye contains different structures that are specially designed to react only to certain kinds of light: cones, which react to daylight, and rods, which react to low (night time like) light levels (Schultze 1866; Stabell and Stabell 2009). What follows is an incremental history of the understanding of the different mechanisms of light and dark vision, cones and rods respectively, and the molecular basis of their photochemical activity.
An interlude on human visual physiology
In the human retina there are about 130 million receptor cells; of them, only four million are cones, the other 126 million are rods. Cones and rods are transducing photoreceptor cells in the retina. A transducing cell take sensory input, say photons hitting the retina, and translates that into an output that is a representation of that input in a form usable by other systems of the body and brain. Cone cells are highly concentrated in an area at the back of the eye called the fovea, or fovea centralis. Human visual acuity is at its best in daylight and at the dead center of the visual field because of the mass of cones in the fovea. As you move away from the fovea the number of cones dramatically drops, and the number of rods increases until in the peripheral field of the retina there are only rods and no cones. The rods handle lowlight and peripheral vision, and are achromatic (i.e. they do not detect color).
An interlude on the history of the understanding of the photochemistry of vision
Vision cognition is handled in steps: first in the eye, with the lens and cornea focusing light onto the photoreceptors of the retina (called ‘retinal phototransduction’); then in the neural tissue behind the retina that transports the received signals from retinal photoreceptors to the brain; and then the finishing touches (the cognitive aspects of the visual process) are applied in the brain proper (Marr 1982; or for an approachable overview of visual psychology generally, see Gregory 1990).
Wilhelm Kühne (Heidelberg University, in Heidelberg, Germany) and Franz Boll (University of Genoa, in Genoa, Italy) discovered that retinal phototransduction involved photochemistry in the mid 1870’s (Stabell and Stabell 2009). In 1877, Boll extracted a substance from the retina of frogs that he called “visual purple” (what we now call “rhodopsin”), and showed that it bleached out when exposed to light and regenerated when left in the dark (Boll 1877). Boll thought this substance was involved in color vision (Boll 1878), but Kühne rejected this hypothesis because he didn’t find any visual purple in the fovea, where, as we’ve already said, human color vision is best. Instead, Kühne hypothesized that visual purple was a Sehstoff—a substance that allowed vision generally, and showed that visual purple wasn’t the only such substance (Kühne 1877-1878; Kühne 1880).