Renaissance: Art, or Science?

Was It Done With Mirrors?

August 3, 2003

Did Leonardo Da Vinci use mirrors to paint the Mona Lisa?  (Photo: CBS)

“I'm suggesting that artists saw these projections. They're very simple to make, and when you make them, they're very beautiful and exciting.”
artist David Hockney

(CBS) When you consider works of genius - the plays of William Shakespeare, the symphonies of Mozart, the paintings of Leonardo de Vinci - there's always a sense of "How could an ordinary mortal create something of such incredible beauty and complexity?"

We can't shed any light on how Shakespeare wrote his plays or Mozart composed his music. But, as 60 Minutes first reported last December, there is a new theory that may explain how Leonardo and the other Old Masters created their masterpieces.

Did they have help? Yes. Was it magic? No. But it wouldn't be entirely wrong to say they did it with mirrors. Correspondent Lesley Stahl reports.

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You can't stand in front of the paintings of the "Old Masters" and not wonder, "How did Frans Hals make that lace seem so real? How did van Eyck make his armor so gleamingly metallic? How did Caravaggio make his faces so expressive and lifelike?"

The theory is they had help with lenses or concave mirrors. If someone stands outside bathed in light, an image can be projected inside onto a wall, upside down. That projected image can then be copied.

David Hockney, one of our best-known artists, believes that the Old Masters used the early technology of optics and kept it secret.

“I'm suggesting that artists saw these projections," he says. "They're very simple to make, and when you make them, they're very beautiful and exciting.”

When set up carefully in a studio, the projection is bright and clear. To demonstrate how Caravaggio might have painted his picture of Bacchus, Hockney, in a film he made for the BBC, arranged a model and projected his image into a dark room - what artists call a camera obscura.

The image was cast directly onto his canvas, and he traced it. It was so much easier than painting from life. Once the artists saw these flattened-out two-dimensional projections, says Hockney, they couldn't resist.

“It's hard to believe that in the 15th century they would say, 'What an amusing novelty, how interesting ... but let's not use that,’” he says.

Hockney comes at this with a practiced eye. Over the last four decades, he's established himself as one of the leading contemporary artists with his drawings, his set designs, his photo collages and of course, his paintings.

But the man who made icons out of Hollywood swimming pools never imagined that he'd be jumping into his own pool of controversy with his new take on the Old Masters.

Once Hockney figured out how the pictures were made, he set out to discover where and when the use of optics began. In his studio in Los Angeles, he built his great wall of hundreds of paintings spanning hundreds of years.

“We did come to about 1420, and realized something happens,” he says.

What happens is a sudden appearance of realism. Before 1420, faces were idealized; immediately after, they were true to life. Before, garments were flat and formless. After, they were vivid and photographic.

Hockney says it started in Bruges, Belgium, one of Europe's great 15th century commercial centers, where that optical look, a photographic look, first appeared in the works of Flemish masters like Jan van Eyck.

“[He was] a painter who knew about optical projections and had looked at them,” says Hockney. “One thing the mirror projections do is project surfaces quite amazingly, especially shiny surfaces. And there's lots of shiny surfaces.”

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What's so revolutionary about what he is saying? The history of art - the history of the Renaissance - is the history of optics.

Needless to say, Hockney and his book about this, called "Secret Knowledge," have rocked the art world, where most art historians say it's bunk.

“All these art historians, not one of them ever took the trouble to look through a camera obscura to see what it was like,” says Hockney.

They don't like the idea that, as Hockney suggests, the Old Masters traced their creations. There is an implication of cheating in that.

But Hockney says they weren’t cheaters, but great innovators: "Not only did they have skills you think you know, they had marvelous skills about optical things as well."

Hockney points to van Eyck's "Arnolfini Wedding." He used to wonder how did the painter paint the chandelier in the picture: “That chandelier is in perfect perspective. So how was it drawn?” He now believes it was created with a concave mirror, and a pencil.

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Walter Liedtke, curator of European paintings at New York's Metropolitan Museum of Art, disagrees with Hockney's contention that the Renaissance started in Bruges and Florence because of optics.

“Isn't there something about those cultures, the fact that they're predominantly urban, mercantile, sophisticated, with a strong middle class,” says Liedtke.

Hockney, however, has debated the cultural explanation with Liedtke and other art historians.

Critics point out that there's little mention of optics or tracing in the historical record, nothing in documents or diaries.

“Yes, but the historical record has no mention of how they painted the pictures either,” says Hockney in response. “The only people who wrote in diaries were highly educated.”

Even today, he says, the artists wouldn’t tell: “They're very secretive. Remember, they're competing in business as well.”

It was also the time of the Inquisition, when mirrors and lenses were associated with witchcraft.

“When Caravaggio is painting in Rome, around the corner in the square, they're burning Claudio Bruni for looking through lenses,” says Hockney.

Hockney is amazed that art historians don't see what he thinks is obvious - the optical evidence right there in the works of such formidable artists as Holbein, Velazquez, and even Leonardo de Vinci.

“Leonardo describes the camera obscura, meaning he tried it out and looked at the pictures,” he says.

Hockney isn't saying Leonardo traced the "Mona Lisa," but that it has the qualities you see in an optical projection, like the soft shadows.

“Whether he used the lens for this, I don't know. Nobody knows. It wouldn't matter. He wouldn't need it, but he'd already seen the wonderful softness,” he says.

Then there are all those left-handed people who suddenly showed up in paintings - Hockney's smoking gun. If you just use a lens alone, left becomes right, and vice-versa.

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Dr. Charles Falco, a physicist at the University of Arizona, and an expert in optics, heard about Hockney's theory and was fascinated.

As a scientist, he thought, "If this is true, I can prove it." The first painting that caught his eye was the 1543 "Wedding Portrait" by Lorenzo Lotto.

“He made a mistake. That's what told me a lens was used. This central pattern of this geometrical tablecloth goes out of focus,” says Falco. “Your eye doesn't naturally see something out of focus. The only way you could see this feature is if you'd seen something with a lens.”

He also found more evidence in the portrait of Cardinal Albergati by van Eyck: “This is one of the rare examples where a preliminary drawing exists along with the painting. I looked at this and said, 'My God, this is good enough to be a photocopy.’”

“So let's see how actually perfect it is," he adds. "I'll blow up the drawing to the same scale as the painting, and when I overlay them, every feature, every wrinkle, every hair is accurate.”

Except for the ear: “It's completely off. What if he bumped his easel when he's doing this? Watch the ear. It's perfect.”

But Liedtke says there are other explanations. For example, he told us van Eyck could have used a geometric grid to enlarge his drawings.

“I think it's simply been taken too far,” says Liedtke. “We now have a kind of optical explanation for realism in Western art. And the, I have to say, the celebrity factor here is really important. If I wrote this book and submitted it to a publisher, it would be sent back to me. But it's a little bit like Jane Fonda's workout or Steven Seagal's Buddhism. You know? It's David Hockney's optics, and so it goes down well.”

He believes that it's remotely possibly that this could have happened, and it probably happened in some cases, but "simply not to the extent that it rewrites the history of art.”

Hockney's optics is a rewriting of the history of art. But he insists it in no way diminishes the artists.

“Actually, my respect for them is more," he says. "If you were given a tracing of a Vermeer, 'Here, now you paint the Vermeer.' Absurd to think that 'Ah, well, that's done and now I can do it.' You can't.”


Master class in 'cheating'

The Times Higher Education Supplement, October 19 2001

By Martin Kemp

Martin Kemp is professor of art history at Oxford University. David Hockney's Secret Knowledge: Rediscovering the Lost Techniques of the Old Mastersis published by Thames and Hudson, �35.00.


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David Hockney believes that the only way the past masters achieved near-photographic quality in some paintings was through optical trickery. Martin Kemp reports


It is so realistic that it looks like a photograph. This comment from an unsophisticated observer looking at a naturalistic picture by a Dutch master of the 17th century or at one of Canaletto's Venetian views is guaranteed to arouse the scorn of an expert. But what are we to think when a draughtsman, painter, scene-designer, maker of photo-works, and inquirer into representation of the sophistication of David Hockney says essentially the same thing of a range of painted images from the Renaissance to the 19th century?


More precisely, what was I to think in June 1999 when Andrew Dempsey, formerly of the Arts Council and Hayward Gallery, suggested that I talk to Hockney about his use of a camera lucida to draw portraits and his idea that historically painters had used optical drawing devices far more widely than anyone had previously supposed?


When I visited Hockney's London studio during one of his visits from California, I found that he was undertaking a form of visual inquiry into the art of the past -  through his inimitable approach to looking and picturing - that was venturing into territory that no art historian had previously dared enter.


In practical terms, he was experimenting with a camera lucida to make drawn portraits. The camera lucida, patented by William Hyde Wollaston in 1806. is a prism-based apparatus that relies on refraction and internal reflection to allow the draughtsman simultaneously to see the subject and the drawing surface. The effect. once we learn to use the device, is as if the image is actually hovering on the surface. Hockney was inspired in his experimentation by his observation that some early 19th-century drawn portraits by J. A. D. Ingres  - then on display in New York and London - showed clear signs of the use of optical projection. As I had come to recognise from my research. the line that results from the tracking of an optically projected contour is quite distinct from that of drawings that are directly ' eyeballed'. (Hockney's term) from the subject. Once one has an "eye'' for the telltale signs, they are relatively easy to spot.


As one of the portrait subjects of the artist's unrelenting scrutiny, between passing clouds of cigarette smoke, I was surprised by the way that he was using the camera lucida. Rather than meticulous and laboured tracing of the projected image, Hockney used it for a form of summary charting, rapidly laying down the key locations in the sitter's facial map, before the expression froze and the features sagged. After some 90 seconds or so of charting, the rest of the drawing, taking an hour or more, was laid in by normal "eyeballing''.


This method opened the possibility that the use of optical devices might not be limited to those artists who paint meticulously "photographic" images and could be extended to masters of rapid brushwork and alla prima virtuosity, such as Frans Hals.


This was not the end of the surprises. From the springboard of the Ingres observations, Hockney was diving into the great pools of naturalistic image-making in the European tradition, from the Renaissance to the 19th century. Driven by an impetuous curiosity and unbounded enthusiasm, he simultaneously began to build his own beguiling devices, most particularly camera obscuras - "dark chambers". with convex lenses or concave mirrors - and to re-examine the visual evidence within historical paintings about how the painters might have achieved their often miraculous ends.

He could not believe that some pictorial effects, such as creased draperies bearing elaborately foreshortened patterns, were simply "eye-balled', without recourse to optical assistance. In an exchange of correspondence, published in part in his latest book, we began to debate the relative roles of the geometry of linear perspective and optical projection in the formation of such images as the globe in Hans Holbein's The Ambassadors and Dutch interiors of the 17th century.


Whereas I looked for evidence in the historian's conventional places - artists' treatises, inventories of studios, accounts of artists at work, surviving instruments and so on - Hockney's evidence was drawn from the visual qualities of the pictures themselves, combined with his painter's question, "how could I do that?" and the strange visual magic of the images formed in his own "cameras". In his definition of visual qualities, he was led to create the kind of startling and telling juxtapositions that are now largely confined to the distant era of "art appreciation". A bit of Ingres drapery is juxtaposed with a traced egg-whisk by Warhol, while a Byzantine mosaic of Christ is set beside a Van Gogh portrait. The two latter images frame his great chronological wall of reproduced images in the California studio.


It would be too simple to say that I acted as the pedantic keeper of the historical conscience in this enterprise. while he dashed creatively into daring realms of historical supposition - but there is something in this characterisation. Hockney's studios in Los Angeles and London became laboratories of visual research and experimental modelling, involving a loose team of diverse talents, not least those of David Graves who chased up historical texts and helped construct the viewing chambers. Charles Falco provided detailed know-how about the nature of lens-based images and how to determine the parameters of the "camera" needed to make a particular picture. Philip Steadman contributed his experimental knowledge of Johannes Vermeer and the camera obscure.


Unexpected artists entered the fray. Caravaggio, master of dramatic narratives painted with no apparent drawing, became a kind of film director, or rather a photographic collagist, assembling his deeply shadowed tableaux from figures projected in strongly foreshortened poses. The concave mirror, which offered greater flexibility and was less dependent on a large "dark chamber,' than the lens, began to play a dominant role. All this continues, and in a sense the book is an intermediate staging post on a great journey.

Did Jan Van Eyck use optical devices? Hockney believes the montage techniques used in the Ghent Altarpiece(1432) are similar to his own in Pearblossom Highway.


Where do I stand as Hockney's claims have become ever more extensive? As he knows, I don't think optical projection was used in all the cases he adduces. I still accord linear-perspective constructions a conspicuous role, particularly for artists who regarded the mastery of geometry as part of their claim to be intellectuals, and tend to give greater scope to the ability of great painters to "fakes, the quality of a camera image through visual memory and incredible acts of painterly sleight of hand. But Hockney has utterly transformed the scope and basis of the debate about mean-ends in naturalistic art for those prepared to cast aside prejudices about artistic "cheating,'. Some cases seem to me to be cast iron, particularly from the 17th century onwards. The suggestion that Jan Van Eyck and his fellow Dutch masters of painted light utilised optical devices is about as convincing as it could be without written evidence that continues to provide the cautious historian with succour.


But to conclude by picking out niggly "nos" and qualified "yeses" seems less important than to acknowledge that Hockney's bigger picture brings home to us how the mainstream of western image-making after the invention of photography went not so much into the specialised practice of "art" but into photography itself, into film and television, and now into all the forms of graphic realism produced by computers.


In a sense, the predominant tone of western painting from Renaissance Italy and Flanders to 19th-century naturalism was indeed "photographic”, if we are granted the licence to use the term retrospectively to denote illusions that aim at some kind of optical matching of light-borne images and the disposition of pigments on a flat surface. In the face of the predominant form of academic art history that seems scared of hard looking, my view is that we should bring on as many David Hockneys as possible.


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The dialogue continues


Extract from letter to Martin Kemp - May 7 2001


Dear Martin,


A few days ago, we made a drawing using optical projections  of figures like the composition of [Caravaggio's] Supper at Emmaus. It is amazingly simple to do. You do not begin with a tableau of four people. Not necessary, not even a large table. Every figure actually occupies the same space in the studio, it is the canvas that is moved around, forwards and backwards (difference only about two feet) and upwards and downwards - also not that much difference.


The actors move only on a straight line from the lens - away or closer - for changes of scale. It is ingeniously simple, and I think the only way optics can have been used for it. It also explains the scale of the hands and even the different scales of the still lives on the table. It would not be possible to set four people at the table this way and photograph them. It wouldn't look like that. It is composed in his head and on the canvas.


This is very exciting, as exciting as discovering the mirror lens. One draws with optical projections - indeed, it is really like Photoshop. If the young users of Photoshop knew all this, they could really make new-looking pictures, although paint is better than printing ink...


David H.

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Additional Material

David Hockney explaining perspective.

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A Review of Hockney's Book from the Guardian Newspaper

In Secret Knowledge: Rediscovering the Lost Techniques of the Old Masters,David Hockney reveals how artists caught nature with lenses and mirrors. It took a painter to show us, says Peter Robb

Saturday October 20, 2001 The Guardian

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David Hockney's Secret Knowledgeis a short and thrilling book with big new things to say about European painting. Hockney has followed his painter's technical curiosity into questions that have mostly been ignored by academic art history. Were the great leaps in verisimilitude achieved by European painters after the middle ages enabled by the use of mirrors and lenses?

Illustrations are a large part of the story here. Linked by an exiguous text that breathes an engaging Boys' Own practical enthusiasm, the pictures create a tension between your desire to linger on the gorgeous images and your eagerness to turn the page and see where Hockney is going. This essay is followed by a very useful little anthology of writings on optics over the last 1,000 years, followed in its turn by an intermittently interesting sheaf of letters exchanged between Hockney and various academics on his research.

Hockney started from a conviction that Ingres used a camera lucida - basically a small prism on a stalk - to draw rapid portrait likenesses on paper. This is not very exciting. The big questions lie in painters' use of the camera obscura to cast images of the real world on to a darkened wall.

Secret Knowledgepresents a series of simple but immensely suggestive technical notes on a series of paintings, especially on the qualities of perspective or proportion or finish that give clues to how they were made. Any medium or technique for making art creates a set of possibilities and limits that bear on the artist's activity at every point and are inseparable from what is achieved. Some of Hockney's best thoughts on the optics implicit in the art are almost thrown away here. The richness and precision that a projected image helped make possible in painting, he remarks, also involved the painter in another kind of tyranny or falsification, that of the monocular view of the single lens, unlike the binocular vision of the human eyes.

He also points out that painters using a camera obscura were likely to have only one small part of their life models in focus at any time. So they worked moving from one small area of the composition to another, during which time the light source - the sun - and the shadows it cast would move, and a sense of time and motion would be incorporated into the finished work. Meanwhile, the painting of complex works with several figures, done part by projected part, would create problems of depth - eyelines that didn't meet, for instance - that enhance the dreamlike vividness and the spatial enigmas of some of Caravaggio's work.

Although Hockney argues that optical devices were used by European painters from the early 15th century, 1600 was the moment when the look of European painting changed. The agent of change was Caravaggio. Some - not all - of Caravaggio's painting uniquely compels you to grope for words in order to describe the optical novelty and disturbing immediacy of the images. They're at once coldly precise, voluptuously real and strangely oneiric. They're certainly alien to the geometrical perspectives of renaissance art.

These paintings were all done in Rome in the decade from 1595, when Caravaggio was part of an intensely scientific and experimentally minded milieu in a household frequented by Galileo, who was then developing the telescope. His patron, Del Monte, owned one of the few copies of Leonardo da Vinci's still-unpublished writings on art and science. Della Porta's book on natural magic, which described clearly and vividly how the camera obscura worked, had begun circulating widely in a new edition just before Caravaggio arrived in Rome. His very first paintings don't yet use this technique (this includes one, the Sick Bacchus, which I think Hockney misreads, since it was done before Caravaggio had access to the new technology of image projection and is a self-portrait painted, as a contemporary wrote, looking at a mirror). Later, when he was on the run, Caravaggio had to abandon the technique altogether.

In Rome, Caravaggio owned and used mirrors and compasses. Contemporaries agreed that he painted directly from life. He didn't draw, and so never worked in fresco, which needed a preliminary drawing. Unlike other painters, he marked out the limits of a few forms, a kind of elementary tracing that left tiny grooves in the wet priming of his canvases, as if to fix the components of a projected image. These techniques were secret because they were dangerous. Science and magic were equally suspect. Della Porta had run foul of the Inquisition, and Galileo would too.

Hockney shows how simply this projection could be made. All Caravaggio needed was a concave mirror and a strong light source, though he might have used more. It will now be hard to deny that Caravaggio, in these central Roman years, painted from images projected like colour slides on to a flat surface. Together with the utterly different Vermeer, half a century later in the Netherlands, Caravaggio is the great instance in favour of Hockney's case. Other painters used optical devices, but Caravaggio and Vermeer made their optics central to the way they saw the world. That is why, in the 17th century, optics in art suddenly mattered.

The delight of Hockney's book is the stimulus it gives us to look afresh at paintings we know well, to think and see for ourselves. His remarks on visual detail are brilliant. I think his larger thesis is essentially right, if not equally pertinent or fruitful in all its parts. New insights sometimes have to fight the tide of a faintly tedious obsession with the minutiae of mechanical technique. Are the portrait sketches of Ingres, let alone the tracings of Andy Warhol, interesting enough for us to care much how they were done? But with Caravaggio and Vermeer and C�zanne - who is the great contrary instance - Hockney strikes gold.

It took a painter to show us.

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A Review of Hockney's Book from the New York Times

David Hockney's 'Secret Knowledge': Optical Illusions
By Bernard Sharratt
The New York Times, December 23, 2001

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"This van Eyck drawing/painting pair . . . gives us amazing insight into specifically how an artist worked 600 years ago. That is, we can see he used a lens . . . in a camera obscura to produce a sketch at one time, then used a lens in an epidiascope to transfer, and magnify, that image onto a canvas at a different time."

So wrote Charles Falco, a professor of optical sciences at the University of Arizona, to the artist David Hockney in July 2000. It was the latest ''sensational'' discovery recorded in "Secret Knowledge: Rediscovering the Lost Techniques of the Old Masters." Indeed, much of the responsibility for the novel claims recorded in this book should surely rest with Falco, though it is Hockney whose celebrated name figures on the cover.

That cover shows part of the long wall of colored reproductions Hockney assembled in his California studio to survey the entire span of Western art -- and van Eyck's painting of Cardinal Albergati (c. 1435) is placed just at the crucial hinge, on the spine, where, according to the Hockney-Falco thesis, Western art took its decisive turn, into the age of photographic realism, 400 years before chemical photography. From the 1430's onward, many major artists, they speculate, secretly used optical devices to make their masterpieces.

The evidence is designed to be overwhelming: 200 pages of lusciously produced images - van Eyck, Caravaggio, Vermeer, Ingres, Van Dyck, Chardin. The book also includes the impressively attentive correspondence over more than a year between Hockney and variously polite, enthusiastic, encouraging but on the whole cautious art historians as Hockney pursued the ever-widening implications of an initial hunch: that the rapid daily portraits Ingres did around 1815 for passing tourists must surely have been drawn with the aid of a camera lucida (patented 1806). Since Hockney realized that he himself could not have drawn such portraits that well, that small, that quickly, he reckoned Ingres at least fixed the main features of his sitters by peering through what is essentially a prism on a stick with a precisely positioned eyepiece, to give the illusion of a reflected image projected onto a tracing sheet.

Hockney began wondering whether other artists might similarly have used optical aids and kept quiet about it. He sought advice from sympathetic experts, including Martin Kemp, erudite authority on the many wonderful but forgotten mechanical aids artists undoubtedly have used, and Philip Steadman, rigorous exponent of the century-old hypothesis that Vermeer used a camera obscura (a kind of dark cubicle with a small hole in the door, through which a lens projects into the interior the scene outside; with enough light outside you can trace the upside-down image on the back wall inside).

Just as Hockney was beginning to put together this wide-ranging book, and the experts were providing more scholarly and practical caveats, the optical scientist, Falco, made his dramatic entrance. Two days after his first visit to Hockney's studio, on March 1, 2000, Falco faxed him:

"I couldn't stop myself from doing some optics calculations based only on my memory of one particular image you showed me. . . . The lens needed would have had a focal length of 2.18 ft (665 mm)."

Two days after that (having, I trust, at least found a postcard of the Lorenzo Lotto painting, a portrait of a man and woman), he faxed again: "There's enough evidence to convince any jury of physicists to convict Lorenzo Lotto of use of an unregistered camera obscura (a jury of art historians is another matter, of course)."

Indeed. That jury might have pointed out that the problem Falco had spotted -- that two parallel lines on a painted table covering go askew to the "true" vanishing point -- could have been caused by a rather simpler device, the humble ruler, shifted a mite carelessly. And that in any case some artists of the period didn't at all mind multiple vanishing points but rather exploited them. And that the preliminary drawing for this very portrait shows no sign whatsoever of being traced from a lens projection.

However, agog with Falco, Hockney hares off to find other askew tablecloths. The corner isn't quite right in Hans Holbein's 1532 portrait of a Hanseatic merchant, Georg Gisze, so, obviously, a tricky problem of lens refocusing. But did Hockney not notice the ostentatiously jokey optical tricks in this picture? A sealing wax trompe l'oeil caption that impossibly seems to hover both on the rear wall and on the painted surface; the sitter's own signature apparently incised into the very wood of the panel behind him; the shelves that go at crazy angles to each other? The picture is a bravura demonstration of all the difficult perspective exercises once listed by Piero della Francesca: a ball, a hanging ring, an obliquely angled and transparent vase. Precarious coins on the wonky corner are meant to be falling off the edge -- to illustrate the sitter's glum motto, visible above his signature, "No pleasure without regret!"

By March 8, the exchange of faxes between Hockney and Falco is already hypothesizing a Newtonian telescope, more than a century before Newton did, in the cylindrical tambour watch on Gisze's table.

By the end of March, Falco is writing, "Anyone arguing . . . that Memling didn't use a lens would be a person too thick to worry about." Since Hockney was by then actually putting the book together, those first 200 pages of glorious illustrations were largely organized and annotated with little beyond such presumptions to support the thesis offered, and with apparently little pause for alternative thoughts. How, Hockney asks, did van Eyck transcribe into two dimensions the exact three-dimensionality of the chandelier in his famous "Arnolfini" portrait, if not with the help of a lens to cast an image of it onto paper? But Hockney is here begging the question, seduced by his own presumption of photographic practice into assuming that van Eyck was "photo-graphing" something he could have visually constructed using the traditional geometry, dividers and compass that enabled cathedral masons expertly to turn two-dimensional diagrams into three-dimensional stone every day. The chandelier is, Hockney might have noticed, remarkably Gothic in design.

Another example: Hockney sees a fleeting smile in the 1580's as almost impossible to capture without optical assistance. Has he forgotten the Elizabethan miniaturist Nicholas Hilliard, in ''The Art of Limning'' (c. 1600), gently warning against falling in love with the smile he so minutely and precisely describes? And paints: Hilliard's smiling Frances Howard is all of 1.26 inches by 1 inch, including elaborate bodice and flowing hair. Sir John Harington, a courtier, said he saw Hilliard "in black and white in four lines onlie set down the features of the Queens Majesty's countenance," exactly recognizable and without even "a Pattern" to help him. Beat that, Ingres!

But the Hockney-Falco prize exhibit is the presumed portrait of Cardinal Albergati, specifically the exact relation between the two images: the painting and the drawing, which is smaller by a ratio of 10 to 7. Falco scanned both images, and rescaled and layered them together in Adobe Photoshop. He excitedly wrote Hockney that there is such an exact matching of features that the one must have been optically traced from a magnified image of the other. The implications are truly critical for this book.

So, scientifically replicating this experiment, and using the same book Falco used, the London National Gallery's "Giotto to Durer," I too got my rescaled digital images to line up pretty much as Falco claimed. But then, commonsensically, I scanned in two other reproductions of the "same" images (from Elisabeth Dhanens's huge "Van Eyck"). I repeated the procedure, and got interestingly different results. I humbly acknowledge my undoubted thickness, but I still suspect that the apparent persuasiveness of any such results could reflect the prior processing (aspect ratios, camera angle, printing and layout options) that inevitably, and variably, enter into all such reproductions. Including those on Hockney's great wall. And accompanying this review.

It's part of Falco's case that van Eyck did the drawing quickly (therefore with handy optical aids) because the cardinal was only in Bruges for three days. This ignores the awkward fact that the drawing is actually silverpoint, hardly suitable for a quick sketch. It would, on the other hand, be normal for a "master-piece," an image to be precisely copied and recopied in an apprenticeship exercise.

Falco's case also ignores the intriguing problem that the portrait may not be Cardinal Albergati at all (scholars argue, as they annoyingly do), but that, if it is, his secretary on that very trip to Bruges, in December 1431, was quite probably a certain Leon Battista Alberti, who four years later published the most influential book ever on painting, "De Pictura." And showed no knowledge whatsoever of van Eyck's alleged use of a mirror lens. Perhaps the cardinal sat in bright sunlight outside a half-shuttered window with "van Eyck frantically rearranging the impatient cardinal until he finally managed to get an image on his pad that was bright enough to use," as Falco imagines. And then neglected to describe the amusing experience to his notoriously inquisitive secretary. The cardinal was thus, presumably, the first of many generations of sitters as well as artists who have actively deceived the gullible public with numerous misleading descriptions and depictions of artists at work in their studios. Umberto Eco ought to write the novel.

So, are the Falco-Hockney claims just fiction, after all?

Perhaps not quite. Falco's one indubitable contribution is to point out that a concave (for example, shaving) mirror acts like a lens and can project an image, given certain lighting conditions. And such mirrors were indeed available in the 1430's, as were the half-shutters and strong light that activate the phenomenon. This is what really got Hockney excited. And I am quite delighted to imagine that van Eyck might have seen something like this enchanting effect. Indeed, if you position a shaving mirror so that it projects the hazy reflection of a far landscape, with upside-down blue sky, you may well find a pleasing resemblance to van Eyck's (possible) contribution to the Turin Book of Hours (the tiny landscape in the Baptism of Christ). Hockney doesn't mention this example, so I offer it in friendly if ironic support.

Despite the dazzling visual feast and Hockney's enthusiastic pursuit, there is actually no point in "Secret Knowledge" where an appeal to optical aids, to explain either large developments or local oddities, seems both necessary and persuasive. What began as an unsubstantiated guess about Ingres has merely been elaborated into a highly marketable myth. Nicely in time for Christmas.

Bernard Sharratt is producing an interactive CD-ROM on the history and architecture of Canterbury Cathedral.

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A letter published in Optics & Photonics News, June 2004