Art historians did not take this lying down. Keith Christiansen, a curator at the Metropolitan Museum of Art, read an open letter to Mr. Hockney testifying that he had gone out and bought a concave mirror at Duane Reade. His verdict? The projection the mirror threw onto his paper wasn't clear enough for him to make a decent drawing. Besides, he added, there is plenty of evidence that artists like Michelangelo, Raphael and Caravaggio had ''no need for fuzzy, upside-down images.'' They made freehand preparatory sketches instead.

Susan Sontag went after Mr. Hockney's ideology of picture making. To say that there were no great painters before optical devices, she said, is like saying there were no great lovers before Viagra. It is a ''very American'' kind of argument. Although Mr. Hockney was born British, she said, in his thinking ''he is one of us.'' To argue that there is a ''direct line from van Eyck to television,'' she said, is to use present-day mass visual culture as the lens through which the past is examined. It represents the ''Warholization of art.''

Linda Nochlin, the Lila Acheson Wallace Professor of Modern Art at the Institute of Fine Arts, was as dramatic as Mr. Hockney. At her signal an audience member brought Ms. Nochlin's wedding dress onstage, a white shift with blue doughnut shapes on it. As evidence that artists can draw patterned cloth without the aid of optics, she compared the dress to a wedding portrait that Philip Pearlstein, ''an eyeballer par excellence,'' had made of her sitting in that dress while her husband slouched next to her in white pants. ''This is what I call scientific evidence,'' she said.

Then the gloves really came off. David Stork, an associate professor of computer science at Stanford University, considered the little convex mirror in van Eyck's Arnolfini wedding picture, the mirror that, Mr. Hockney suggests, van Eyck could have flipped over and used as an optical device. First off, Mr. Stork said, a mirror of that size would never have worked. To get a lens that would ''hold Arnolfini, his wife and dog,'' he would have needed a huge mirror, sliced from a sphere seven feet in diameter.

And that is just the beginning of the trouble. If van Eyck had used the lens in a camera obscura, he would have had to paint upside-down, Mr. Stork said. Then there is the lighting problem: the projected image in a camera obscura would have been too dim. ''To mimic the conditions indoors on a gray day in Bruges,'' he said, would require hundreds of candles, and then, even if the artist were to survive the fire hazard, ''the color looks wrong.''

Ellen Winner, a professor of psychology at Boston College, kept Mr. Hockney on the ropes by showing some excellent, optically exact drawings of rearing horses. They were made by a 5-year-old autistic child named Nadia, who had seen only pictures of horses standing still. If an autistic 5-year-old can do this, Ms. Winner said, then ''I would argue that a Renaissance artist could do it, too.''