Piero's Light

by Larry Witham

  A nineteenth-century generation of people such as Milanesi in Italy, Eastlake in England, and Burckhardt in Switzerland was important for understanding Piero today. The same could be said of another figure in this generation, the German scientist Hermann von Helm­holtz, who was born in 1821 and did his major work in the years, for example, when Piero’s paintings were entering the National Gallery in London.

  Much about the nature of color had been understood before Helm­holtz came on the scene, but he would add to this knowledge of how the physical eye and brain “see,” and he would also speak to how these scientific principles applied to making art and seeing art, paintings in particular. By Helm­holtz’s time, the theory of light as a wave was back in fashion, especially for the field of physics, which was finding that wave frequencies explained a good deal of natural phenomena. Furthermore, the realm of electro­magnetism had been discovered, an omni­present world of energy in wave­lengths, and now light and color were understood to be a particular part of this electro­magnetic spectrum. Indeed, without the human eye to detect this “light” spectrum, there would be no light at all, just darkness and electromagnetic waves of various lengths.

  Helm­holtz also worked on the foundations of a new theory of how the eye retrieved the different wave­lengths of light. For the eye to find and mix the so-called primary colors in the light spectrum—red, green, and violet—it needed “particles,” now known to be cells shaped like cones and rods (and thus their modern anatomical names). This matrix of cells begins the brain’s process of distinguishing dark and light and mixing the primaries to produce all other colors.

  Much of this broader knowledge was based on the original work of Isaac Newton. As Helm­holtz knew well, however, Newton’s theory of the light spectrum was not escaping all protest. The most famous and singular protest came from the great literary figure and amateur scientist Johann Wolfgang Goethe, the German poet. With his rhetorical powers, Goethe questioned whether a mathematical and mechanical explanation of color tells the real story of human psychological perception, a polemical argument he made against the Newtonian party in his Theory of Color (1810).3

  Resolving this debate would require more knowledge about the biology of vision, and this was the accomplishment of Helm­holtz. Taking the Newtonian side, he went on to invent the key medical optical instrument of his century, the oph­thalmo­scope. “I had the great pleasure of being the first man ever to see a living human retina,” he recalled.4 In his popular lectures, he used the new findings in optics to analyze the craft of painting. And he gave due credit to Renaissance artists for discovering all the illusionist principles of representation well before science understood the laws behind them.5

  In his Handbook of Physiological Optics (1867), however, Helm­holtz surveyed other facts of nature that were too deep for artistic manipulation. He moved beyond the older descriptions of the eye as a camera obscura, now explaining its defects and its powers. As to the first, light and color can be distorted by the fibers and liquids in the eye. These distortions were compensated for by extraordinary strengths also found in the eye, especially the eye’s focal point, which has such a strong acuity that it can produce the illusion of sharp vision everywhere by the eye’s rapid movement. Pictured now as machinery, the retina at the back of the eye was a complex detection device. Its layers of nerve cells interacted with light, the “visible” part of an electro­magnetic spectrum, a reality of energy waves that permeate the world.

  Under electro­magnetism, Helm­holtz explained, the cells in the human visual system behave in patterns. When excited by light, they let off an impulse. After that, they fatigue and recharge. Here was an explanation for afterimages: after the eye sees red, red receptors are fatigued, so the eye sees green as the green receptors dominate. Even more remarkable, Helm­holtz said, is how the visual system, bombarded by every kind of electro­magnetism, nevertheless produces a lawful perception of color and form.6 Of the three basic qualities of color—hue, intensity, and brightness—the third (also called luminosity) was turning out to be the most important for visual perception.7 This was another point already made during the Renaissance, when Alberti argued that the dark/light contrast, even before color, was the heart of artistic representation.

  Going well beyond Alberti, Helm­holtz finally proposed a mechanical source for beauty itself. Renaissance belief, again typified by Alberti, had located beauty in both the innate mind and in a tran­scen­dent realm. Helm­holtz was familiar with such a tradition, for even in the natural sciences of his day, as exhibited by the late Goethe’s popularity, there was a desire to find vital forces, even metaphysics, by which nature speaks to human intuition. Helm­holtz called himself a Kantian, agreeing that the mind partly invents the laws of nature. Finally, however, his goal was a thoroughly mechanistic science. Vision, perception, and even beauty might have purely mechanical explanations. In this he anticipated the field of neuroaesthetics, which would study how cells and modules in the brain perceive visual art—and even beauty itself.

  On that topic, Helm­holtz went as far as nineteenth-century science allowed. He suggested that beauty is based on comfort to the eye, whereas glaring colors cause fatigue, not aesthetic pleasure. For instance, he said, “a certain balance of colors is necessary if the eyes are not to be disturbed by colored afterimages.”8 The artist must cooperate with nature in this pleasing effect, for while a “beautiful painting” must be vivid, its success is “not just in reproducing colors, but in imitating the action of light upon the eyes.” In fact, paintings can have advantages over the real world in producing beauty, he suggested. They can tone down the power of nature, give it a focus, and represent the world “without injuring our eyes or tiring them by the harsh lights of reality.”9

  After Helm­holtz’s achievements in optical biology, the puzzle of vision was handed back to physics, whose golden age began with the twentieth century, when new discoveries included the atom, the x-ray, the quantum, and finally Einstein’s theory of relativity.

  All this was bubbling up from scientific laboratories around the same time that modern art connoisseurs, from England to Italy, were discovering Piero’s “color-form,” “unemotional” stillness, and “plasticity” of design, and mathematicians were re-evaluating his accomplishments in mathematics and geometry. It was also a time when, in fact, new and weird geometries had begun to fascinate a group of painters to be called the Cubists. Some of them searched for a new “painter’s geometry” and, though futile, desired to paint the “fourth dimension.”10 None of this actually led to the application of new scientific knowledge to age-old painting techniques.11 But the artistic ferment did parallel some astounding new discoveries in physics—discoveries that would alter our understanding of not only art, but even of how the universe operates.

  This new vision of the cosmos began with a challenge to the reigning wave theory of light.12 It was an unwitting challenge, and yet it inexorably arose in 1900, when the German scientist Max Planck was experimenting with blackbody radiation (heat with no color spectrum), the uniform heat typified by an oven. Planck discovered that heat, which is the energy given off by electro­magnetism, does not change smoothly, but rather in small leaps, as if in packets of energy. This implied that energy comes in particles; thus was revived the corpuscular theory of light. On closer inspection, light was found to behave as both a wave and a particle. Eventually, light would be spoken of as a wave frequency, but also as a fast-moving particle called a photon.

  Light became still more enigmatic when Einstein proposed that the speed of light—that is, the speed of a photon—is the only absolute constant for humans in the universe. In the theory of relativity, at the largest scales of the universe, space and time were “relative” to the human subjects, depending on how fast they are moving in the universe, with the speed of light as the maximum speed possible. In Platonist terms, the space and time of the universe is in flux, regulated by only one type of signal to
human perception: the photon, which unveiled the world by its light and its rate of motion. Yet the photon has not yet revealed its own mystery. It has a particle-wave duality that remains elusive to mechanical descriptions. It can be understood only by tapping into the Platonist realm of mathematics, which can use numbers to make a relativity universe comprehensible.

  The ancient Platonists had given light a metaphysical role in the universe, and now it seemed to be no less important for how moderns understood their perception of reality. By uniting the new knowledge about light and atoms, physics explained color at a deeper level, the level of atomic particles. When photons hit any object, all of which are made of atoms, the atomic particles make jumps in energy. Some of these jumps emanate a wave in the visual spectrum, thus producing the effect of the object’s “color” on the human retina. Any molecule that operates this way, absorbing a color frequency or emanating one, is a pigment. In his Baptism of Christ, for example, Piero painted the tall hat on one of the men in red tempera. It appears red because the pigment absorbs light frequencies of green and violet, emanating mostly a red frequency.

  From the point of view of light and color, the world was turning out to be a strange place indeed. In truth, it is a dark and color­less world, at least until the energy particles and waves reach an eye. In the old Platonist system, light was the source of the world; in modern physics and biology, the same could be said almost exactly. How the brain turns these quantum effects into mental apprehensions of color, form, and beauty would await a next leg of the scientific revolutions. Physics handed the problem back to biology, and it, too, would evoke a central theme of Platonism: essences and change.

  In the Platonist traditions of Alberti and Piero in the Renaissance, the world might change, but behind it God had fixed a range of unchanging essences, from the revealed truths of the faith to the geometries of music, proportion, and beauty. The next modern revolution in biology, characterized by everything from the modern Darwinian synthesis of the 1930s to the foundations for modern genetics and neuro­science after the 1950s, would lead to a new understanding of what changed and what stayed the same. As to a benchmark, the fifties was also the decade that Kenneth Clark wrote the first modern English book on Piero, and Ernst Gombrich began proposing, if just barely, a biological approach to art in his worldwide bestseller, The Story of Art.

  The new Darwinian biology looked at nature operating over time, so naturally change was the issue of most interest. Under the Darwinian view of the world, it was popular to see change and progress everywhere, especially in culture and society, where knowledge, technology, books, or education could significantly alter a society in a human generation or two. As appealing as the marvel of rapid progress could be, however, it was not going to be fully applicable to the fundamental biology of the human brain. The human brain, as science generally acknowledges today, has not essentially changed since the rise of modern humans on the Pleistocene savannahs of Africa.

  For the science of perception, this means that the brain has retained a high degree of constancy. In the timeline of human evolution, the physical powers of the brain were virtually identical in Piero della Fran­cesca’s era to what they are today. People in the twenty-first century, in other words, are equipped to see the most basic elements of Piero della Fran­cesca’s artworks just as he and his contemporaries had perceived them, at least in terms of brain function.

  Naturally, many art critics and theorists reject the evolution of the brain as the fundamental basis for human tastes in art. They prefer to argue that social psychology, culture, and language are such powerful external forces that they override any biases, or constraints on perception, dictated by the physical brain structure. Indeed, in postmodern thought, the idea of “nature” itself is denied, and while biology (made up of molecules, cells, and organisms) may be real, it is a lesser force on human behavior than the “socially constructed” world of culture, language, and ideology.

  In the challenging of science, postmodern thought—and even some philosophical theology—has had one great crack in the door: science itself is still struggling to solve the philosophical problem of distinguishing the physical senses from the “mind.” In Piero’s day, the Platonist answer to this puzzle had been the existence of the soul, which was separate from matter and was able to perceive tran­scen­dent things amid the flux of the physical world. This was a view, borrowed especially from Greek thought, that was deeply embedded in Christian belief. Today, neuro­science has preferred an alternative Greek idea, which is the materialism of the mind. Still, the problem of explaining how the mind, with all its subtle powers, emerges from the vibration of atoms and cells remains one of the greatest enigmas of human knowledge.13

  The mechanics of “seeing” has proved easier to resolve than explaining the full breadth of human “thinking about” a visual experience. One helpful fiction about vision, which had helped as long as it lasted, was the belief that images landed whole on the retina, like movies on a screen. But more precisely, and to the contrary, the retina receives so much data that nothing is really “seen” until, somewhere back in the brain, the data is sorted through and organized. In short, it is the brain that finally sees, and the noble eye is merely a window and lens.

  After the data of the light gets through the window, it is transmitted between cells as electrical and chemical signals. These move up the line to the visual cortex, an area of the brain at the back of the skull. The signals are further distributed to an estimated dozen more areas. At still another mental level, all this information is integrated: the “mind” distills the essence of the visual impressions, which meanwhile are still coming in almost as rapidly as photons can fly and cells can fire.14 Amid this incredible overload of sensory data, the mind does something remarkable: it compares the visual stimuli with “memory,” and from those memory banks it can identify and evaluate what is being seen in the world.

  Equally remarkable, the workhorse of this event is the nerve cell, called a neuron. The brain contains a hundred billion, which in turn send a thousand trillion neural signals among themselves.15 In this biological matrix of the brain, the visual system is perhaps the best understood of the sensory areas.16 This is due to the clarity of its anatomy, beginning with the eyes and ending in the visual cortex. Along the way, the neurons accumulate and distribute visual information in a hierarchy of importance, serving the needs of the brain, of which accurate perception and physical survival is first and foremost in the evolution of the modern human.

  According to evolutionary theory, the visual system has developed many remarkable efficiencies. For one, the eye’s tiny focal point expands its neural reach extraordinarily widely into the brain. For another, neurons specialize as “feature detectors.” Neurons in the retina turn each other on and off depending on whether light hits a center or “surround” area. Neurons do the tricolor mixing of light as well. Up the line, “opponent” neurons oppose specific colors against each other, such as blue against yellow or red against green. Here is found a solution to the long-festering debate on the origin of color, pitting Aristotle, Goethe, and the field of psychology (which favors the opposition theory) against the Newtonian tradition and its allies in physics (to whom color is simply electro­magnetism).17 Both views are correct in their own way. The mixing cells and opponent cells integrate as a unified act of perception, producing a mystery, as some neuro­scientists admit, much like the way a photon is both a particle and wave.18

  What does this mean for art appreciation or for the practice of painting per se? For viewers of art, it is enough to know that the mixing and the opposing systems work together, the end result being the pleasure of color in its constancy and its shifting variety. For the practicing artist or scientist, the two systems provide handles on two contradictory realities in color. Under the light spectrum, the three primaries are the basis for mixing the purest possible paint colors, and yet at the same time we have developed the so-called color
wheel, which reveals that certain colors predictably oppose others: blue opposes orange, green opposes red, and violet opposes yellow. Juxtaposed, these oppositions are visually exciting and flashing. When mixed, however, they produce browns and grays, which are called neutral colors on the artist’s palette. The light spectrum itself cannot explain this quality of contrast and opposition. Something must be happening in the human visual system, where color arises from both mixing and opposing, an inherent mystery to color perception.

  Despite such enigmas, the biological structure of vision is revealing many of its remarkable functions. Farther in, from the eye toward the visual cortex at the back of the brain, feature-detection cells increase their specialization. By their orientation, they detect edges, lines, curves, contrasts, and motion. They also apprehend the most basic feature of light itself, which is luminosity, the relative contrast of light and dark, which now is understood as the most fundamental visual perception. Judging luminosity produces form and depth, allowing the brain to move through the world. The perception of color is overlaid on luminosity, another feat of integration by the brain that remains a mystery.

  In explaining this entire process—first specialized detection, and then integration—some scientists have emphasized the behavior of modules of the brain. The modules each handle an aspect of perception, such as edges or color contrasts. Related to art, therefore, different modules of the brain would be activated in the presence of different artistic styles. This way of seeing the brain, called “functional specialization,” is an important theory in neuroaesthetics, which studies “the neural bases for the contemplation and creation of a work of art.”19 By analogy, functional specialization sees the brain as a parallel processing machine, a term used in computer science when a problem is broken into separate parts and then united later. Nothing is passive in this process. The cells and modules are constantly receiving and sending signals.