OPTICAL ALLUSION: Does the mirror in van Eyck’s Portrait of Giovanni Arnolfini and his Wife hint at a trick of the trade? © The National Gallery, London

The streets of Greenwich Village are generally deserted at dawn. But on December 1, 2001, a crowd began to gather before sunrise. By nine that Saturday morning, nearly a thousand people had formed a restless line, hoping to gain admission to one of the most contentious face-offs of the new century.

“Smackdown!” trumpeted one reporter. But this was not a sports match. It was a scrimmage of intellects in two realms that rarely catch the public interest: optical physics and Renaissance painting.

Just four weeks earlier, British-born artist David Hockney had come out with a book that was creating something of a stir. His Secret Knowledge: Rediscovering the Lost Techniques of the Old Masters (Viking, 2001) proposed that artists as early as 1430 used optical devices to help them create their remarkably detailed and realistic paintings—works that even today look almost too perfect.

Yes, Hockney said, van Eyck, Leonardo, Michelangelo, Holbein, Campin, Lotto, Caravaggio, Bellini, Raphael—and later Velazquez, Vermeer, Rembrandt, Ingres and others—projected images of their scenes onto canvases using concave mirrors and crude “cameras,” then traced or painted over them. Moreover, he claimed, the earliest practitioners were furtive about it, treating the optical techniques as a trade secret while everyone marveled at their uncanny draftsmanship.

What? Old Masters surreptitiously using the precursors of photography? Wouldn’t that be tantamount to cheating?

Not really, said Hockney, noting that artists have always sought tools to improve their work and that only a master can turn a tracing into a masterpiece.

Then why all the fuss? Cultural critic Susan Sontag explained it adroitly. “If Hockney’s theory is correct,” she fumed before a standing-room-only crowd at the Greenwich Village summit, “it would be a bit like finding out that all the great lovers of history have been using Viagra.”

Sontag and 30 other cognoscenti had been invited to spar at the NYU-hosted symposium—where, ArtKrush magazine reported, “things got delightfully out of hand.” Painters, photographers, art historians and curators spoke their minds, as did psychologists, architects, physicists, neuroscientists, philosophers and all manner of other scholars. The two optical physicists played a pivotal role: Charles Falco of the University of Arizona, Hockney’s longtime scientific ally; and David Stork of Stanford, invited at the last minute to critique Falco’s arguments.

It was the highbrow pillow fight everyone had hoped for, but when the feathers settled, the big question was still unanswered. Hockney soon “got a headache” and went back to painting. Yet the debate raged on, spurred largely by Stork’s continued probing. In feisty exchanges on the web and in formal presentations at more than a dozen conferences, including one at Stanford last spring, he became the leading critic of the claim that Early Renaissance artists had a secret method for making their paintings look real.

THE ARTIST AND THE PHYSICIST: Hockney (above) shocked art lovers by proposing that some 15th-century masters covertly traced projected images. Stork rebuts the theory from a technical perspective. Christopher Felver/Corbis (above); Courtesy David Stork (left)

The notion of the Old Masters relying on more than talent, training and a good eye for detail hit, in the words of USA Today, the “optic nerve” of not a few individuals. “Most people, it seems, prefer to envision their artistic heroes as superhuman draftsmen, capable of rendering ravishingly accurate anatomies or landscapes or townscapes through sheer inborn or God-given talents,” New Yorker columnist Lawrence Weschler noted wryly after his January 2000 article introducing the Hockney/Falco theory provoked an avalanche of mail.

David Stork wasn’t perturbed. “I was intrigued,” he says. “At first, I thought, ‘What a clever idea these two have come up with.’ I was even somewhat predisposed toward believing it.”

Stork, a consulting associate professor of electrical engineering and visiting lecturer in art history, coauthored Seeing the Light (Wiley, 1986), a well-regarded textbook on optics in the arts. He is chief scientist at Ricoh Innovations in Menlo Park, developing “intelligent” software, and something of an artist himself, using Late Renaissance techniques to create optically distorted images.

Stork wasn’t out to defend the “honor” of Early Renaissance painters when he agreed to critique Falco’s arguments. “I certainly wouldn’t think much less of such artists if they had, in fact, used optics. But I did think it was important to get the scholarship right,” he says.

“It was only after finding persistent flaws and simpler explanations that I started to doubt—and then seriously doubt—the theory.”

To show that 15th-century artists used concave mirrors as optical projection devices, Hockney and Falco focused on one of the most famous convex mirrors in all of Western art.

At the center of Portrait of Giovanni Arnolfini and his Wife, painted in 1434 by Jan van Eyck—the “father of oil painting”—is a mirror that has puzzled art lovers for centuries. It’s a shiny hemisphere, bulging toward you rather like the tummy of the (ahem, pregnant?) Mrs. Arnolfini. What makes this mirror so interesting is that if you look into it, you see not only the backs of the wealthy Italian merchant and his young bride, but also two figures who must also be in the room, facing the dour-looking newlyweds.

One of them, some art historians say, may well be the Dutch master himself; the other may be his assistant. By rendering the painting in this way, van Eyck cleverly implies that you, as the viewer, are within the scene, too. For you could not be looking at a mirror that reflected back the entire foursome without standing in the room yourself! The Arnolfini mirror on one level thus becomes a philosophical statement about the nature of reality, the seen and the unseen, and the multiple ways in which phenomena can be perceived and interpreted.

Hockney and Falco discerned yet another level of meaning. The enigmatic hemisphere, if reversed, would resemble the concave mirrors purportedly used to cast images onto canvases. It might, in fact, be the one van Eyck used to create this very work. Could the master have planted a sly reference to his trick?

One way to determine whether the famous bulging mirror in the painting could have been turned around to function in this way, Stork reasoned, would be to look at focal lengths.

The image on a mirror is simply light from the reflected objects. If the mirror curves inward, those light rays (the image) bounce off to converge at a single point somewhere in front of the mirror. The distance between the mirror and this point is the focal length. The light rays keep projecting—with the image now inverted—until they reach the canvas or some other surface.

To calculate the focal length of the mirror depicted in the painting, Stork compared the heights of the teensy Arnolfinis in the reflection and the regular Arnolfinis in the portrait. The answer: about 12 centimeters.

If turned around, could this beachball-sized bowl have done the job?

No way, says Stork. He compared the sizes and positions of the painted Arnolfinis with the estimated sizes of the real-life couple and the dimensions of the room, and found that a concave mirror would need a much greater focal length, almost 60 centimeters, to project an image large enough to fill van Eyck’s canvas. The fragile glass sphere from which it would be cut would have to be huge, about 7 feet across—a technical impossibility at the time, he says.

A separate test was to see if objects in the painting were rendered in geometric perspective, which Hockney and Falco cited as evidence of tracing. For this analysis Stork chose an ornate chandelier suspended over the newlyweds’ heads, and plotted out the various lines of perspective. If the chandelier had been projected and traced, the lines would recede like train tracks in the distance, meeting at a “vanishing point” along the horizon. Instead, they jutted in all directions like pick-up sticks. The chandelier Hockney proclaimed “in perfect perspective” was not in perspective at all.

Probing further, Stork reasoned that if the Arnolfini scene had been painted even partially upside down from a projected image, then some brush marks might reveal this through the effects of gravity. He analyzed infrared reflectograms at the National Gallery in London, where the portrait hangs, and concluded that all discernible brush strokes were made right-side-up.

“The evidence clearly points to the fact that van Eyck could not have used a mirror to create this painting,” Stork says.

It was a different assertion that really ruffled his feathers. The very first painting made with the mirror technique, said Falco, was Robert Campin’s 1430 Merode Altarpiece—a work to which Stork was particularly attached.

“I was stunned to hear Falco claim that,” Stork says. “I knew the painting well from having written a paper on it as an undergraduate [at MIT].”

Shifts in perspective suggested to Falco that Campin had moved and refocused a concave mirror around a studio scene in order to paint the piece in successive sections. While Stork, too, detected the shifts, he countered that “refocusing a mirror, which would have to have been done absolutely perfectly, would have been too complex a process.” He proposed a simpler source of the shifts, based on connecting lines with a ruler. Using paper and pencil, his 9-year-old daughter reproduced the effect on her first try. “This seems a much more natural explanation,” he says.

There are no historical records before 1598, notes Stork, of any concave mirror being used to project an image except through burning. On this, Hockney agrees. The artist attributes it to a conspiracy of silence among painters using the secret technique; the physicist rebuts vigorously from several angles. “Wouldn’t the makers of such mirrors have every incentive to advertise them?” Stork asks. “Why wouldn’t the subjects who sat for portraits mention such devices, even in passing in personal letters? After all, the mirror setup included a large dark tent and very bright lights, and was imposing. A concave mirror would have been an expensive proposition on the budget sheet. . . .”

There are no records, says Stork, because there were no concave mirrors capable of projecting an image onto a canvas. Concave mirrors existed in the Early Renaissance, but technology limited their quality and focal length.

“If you look at the concave side of a shiny teaspoon,” he explains, “you’ll see your image upside down. This real inverted image is indeed projected into the space between the spoon and our eye—but in the context of the Hockney debate, the technique demands a different form of projection: the projection onto a screen, such as canvas, paper or oak support, just as a movie projector casts an image onto the movie screen.”

Sara Schechner, an expert in Renaissance optics at Harvard, backs that up. “Close inspection of surviving mirrors in museum collections,” she says, “shows that the instruments of the Early Renaissance were simply too crude to project lifelike images.”

For two years now, Stork has presented his findings in journals and at conferences. In April, he addressed physicists in a jam-packed colloquium at Stanford. In May, he spoke at a symposium at NASA-Goddard in Greenbelt, Md., where several prominent supporters of the Hockney theory said they found his chandelier argument convincing.

“Stork’s evidence is compelling,” says neuroscientist Christopher Tyler, associate director of the San Francisco-based Smith-Kettlewell Eye Research Institute. Tyler, who also rebutted Falco’s claims at the Greenwich Village symposium, teased out discrepancies of perspective in Lorenzo Lotto’s Husband and Wife (1543), which Hockney and Falco dubbed their Rosetta Stone because the pattern on a cloth between the couple seemed to display especially vivid optical effects, such as a blurring of focus in receding portions.

Hockney has withdrawn from the controversy, but Falco (who politely declined to be interviewed for this article) continues to defend the theory in an increasingly hairsplitting web debate at a site created by the “smackdown” organizers.

At this point, Stork thinks he’s essentially made his case. “I wouldn’t be so irresponsible as to claim that I’ve disproved the Hockney theory,” he says, “but the burden of proof now rests with the theory’s supporters. I feel I’ve rebutted all their substantive claims. If optical projections were used in the Early Renaissance, then [the proof] is unlikely to come from the evidence we’ve been given so far.”

Still, the issue has not been put to rest. In mid-November in Ghent, Belgium, historians of art and science will gather yet again to sift and weigh the data.

Even Stork feels compelled to keep the issue and others like it out on the drawing board.

“I want to help bridge the sciences and the arts,” he says. “More important, I want to know the truth about the art I love. Optical analysis can help us see more sensitively, more acutely. The Arnolfini chandelier looks entirely different to me now. It’s really quite amazing how van Eyck could make David Hockney, one of our most celebrated artists, think the chandelier is in perfect perspective when in fact it’s not. Van Eyck has done this, it turns out, with his patient talent and oil paint, not mirrors and optical projections. We can appreciate his achievement more fully in light of these technical analyses.”

Hockney said it his own way at the close of the NYU smackdown: “The paintings . . . are absolutely magical.”

He’ll get no argument on that.

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