Sculpture and painting have been facing off for about five centuries now. Beginning about 1500 in Italy, there was even a kind of official smackdown, called the Paragone (the "Comparison") staged between vocal supporters of the two camps. "Only painting can render light," said one side. "Only sculpture can show a figure from all sides," said the other. "A sculptor has to be strong as an ox -- and at least twice as smart" versus: "Mixing paint is for limp wrists." By now, however, any referee watching the whole 500-year match would have to give the prize to painting: Say "art" and most people first think oil paint on canvas.

The two art forms are going head-to- head right now, in Washington -- with painting taking its usual early lead.

As of today at the National Gallery, there's one week left to see "Thomas Gainsborough, 1727-1788," the latest in a long line of solo shows devoted to the celebrated English painter. Visitors will be crowding into the West Building's elegant, skylit upstairs spaces to get a last look at his popular portraits.

And today is the public's first chance to see the National Gallery's terrific "Jean- Antoine Houdon (1741-1828): Sculptor of the Enlightenment." Houdon may be the greatest portrait sculptor of all time -- he's responsible for iconic images of Voltaire, George Washington, Thomas Jefferson, Ben Franklin and many other of his era's superstars. Yet this is the first comprehensive exhibition of his work. And even at the National Gallery, where the sculpture curators recently launched a bright new suite of galleries to finally do their holdings justice, you have to head into the windowless bowels of the building to catch the sculpted splendors of their temporary Houdon show.

You may have barely heard of Houdon. (Or heard his name pronounced: It's something like "oo-DONG," but with that final "g" left silent.) He's probably not on the radar for many of today's painting- bred art lovers. But this is nevertheless an exhibition worth making a special effort to track down. It is huge fun to watch how Houdon managed to make inert clay, plaster or marble carry the precise look of living flesh. And then to try to figure out how he imbued that flesh with a spark of personality.

There's not much else to Houdon's life and career. He was born into the Parisian middle class, then applied himself throughout his long, comfortably bourgeois life to accumulating prestigious connections and commissions and making them pay. He had substantial business savvy. He was one of the first sculptors to control the reproduction of his works, shaping them first in clay and then offering them up for sale in terra cotta, plaster or marble versions. (Most great sculptures, before and after Houdon, have started out modeled in clay, not bashed straight out in marble.) But Houdon's commercial success couldn't have come about without a base of sheer artistic skill and diligence.

The weird thing is that to reach the very summit of his sculptor's art, Houdon had to steal a painters' trick.

Like the very best sculptors who came before him, Houdon was spectacularly good at rendering every detail of firm flesh or sagging skin. He molded masks direct from sitters' faces -- sometimes from famous corpses, too -- and even measured his patrons with calipers to make their portraits stunningly true to life.

As a young French prodigy on scholarship in Rome, Houdon studied cadavers and then worked up a kind of Visible Man anatomic model -- known as an e{acute}corche{acute} -- to guarantee that his finished sculptures of saints would have each ripped muscle in its proper place. Casts of Houdon's model have been used ever since to teach anatomy to artists. His classically inspired nudes, both male and female, are also close to anatomically complete, even in some shocking details.

But from his own day until now, everyone has said that, however good he was at flesh, the secret to Houdon's genius was in the way he modeled eyes. For the first time, the eyes in sculpture seemed to glitter with the animation that they have in life -- or, more accurately, as they do in a great portrait painting.

Look at Houdon's eyeballs. They don't have the creepy, smooth, featureless surfaces you see in most of the classical statuary that has come down to us. (Or as Houdon renders them whenever he is treating classical myths or allegories.)

Houdon's busts also avoid the kind of line drawing of iris and pupil that you see scratched onto the eyeballs of many Renaissance sculptures.

In Houdon, the hole of the pupil is rendered by boring into the sculpture's pale surface, so light cannot seep in and the pupil stays deep black. The iris is rendered by carving out a shallower doughnut all around the pupil, so that a lighter shadow is cast into this area to capture its middle tone.

Houdon even seems to distinguish blue eyes from brown, by excavating the ring of a blue iris less deeply and thus letting more light reflect into it; a series of finely feathered striations sometimes fan out from Houdon's pupils, seeming to render the real-life striations that are more prominent in blue irises than brown.

And now take one final, close look at any eye in any portrait by Houdon.

In each case, a tiny stalk of clay or plaster or stone -- looking like a grain of rice or a jimmy from an ice-cream sundae -- reaches out from the hollowed surface of the iris up to the level of the rest of the eyeball. From too close the detail makes no sense at all; it makes it seem as though all of Houdon's sitters suffer from ocular warts. But stand just far enough away (far enough for small details to start to blend, but not so far they simply disappear from view), at just the right spot (so you see only the tip of that sculpted mote in the portrait's eye), by just the right soft light (so it grazes the surface of the sculpture without casting harsh, confusing shadows -- which are an inevitable problem in this artificially lit show) and suddenly, the eyes on Houdon's figures come alive: The brightly lit tip of that sculpted "stalk" now reads as a highlight glinting on the wet surface of a living eye, hovering above a glowing iris that in turn surrounds a penetrating pupil.

Painters had figured out the crucial importance of capturing such highlights in their portraits several hundred years before Houdon came along. (Photographers are still taught to retouch highlights into eyes whenever a portrait's lighting doesn't happen to produce them naturally.) Among sculptors, however, Houdon was the first to realize that his medium could produce the same effect: He realized that sculpture, like painting, could be about evoking the optical effects of nature, and not just about reproducing its real 3-D shapes and surfaces. An oil painting in this exhibition shows the famous Houdon sculpting in his studio; it overdoes the highlights in his sitter's eyes, almost as though it were trying hard to signal Houdon's special innovation to its viewers.

When it works, the full illusion of Houdon's eye technique is spectacular. At the National Gallery, the 1790 sculpture of the unprepossessing, toadlike Jacques Necker, head of finance under Louis XVI, is set into a corner where all the factors come together to make Houdon's effect succeed. We are led to read the financier's eyes as fully liquid and alive, without ever noticing just what artificial features in the carved, dead marble have made them come to life. And that life then seems to ripple out into the curls in Necker's hair, the swirl of his cloak and the ruffle of his shirt front.

A single, strikingly illusionistic detail -- such as a perfectly modeled eye -- can help us suspend our disbelief throughout a work of art. And once we see an entire portrait as strikingly realistic in its details, we tend to assume that it's an accurate depiction, too.

Houdon's imaginary busts of long-dead heroes like the playwright Moliere or the fabulist Jean de La Fontaine strike us as being every bit as true-to-life as the busts of people he saw in the flesh.

Sculpture before Houdon may have managed to suggest a sitter's personality, but mostly by accumulating separate surface clues to character: A furrowed brow, an absorbed look, a brooding mouth. Houdon, on the other hand, seems to actually imbue a bust with the personality; character seems to come alive within his busts, rather than sitting there on the surface waiting to be doped out.

I think that it's this general liveliness, and the lifelikeness that it implies, rather than the evocation of specific traits of character, that makes an Houdon sculpture work. It becomes a kind of blank screen specially adapted to accepting all the different readings of character that we project onto it.

In painting, it's well known that it takes a viewer's searching mind to make a two-dimensional mess of paint read as some specific three-dimensional scene. That "beholder's share" counts for just as much in any Houdon bust: We first read the lump in each one of its eyes as surface sparkle, then we read that wet-eyed figure as alive, and then we fix on the character we spot living in it.

I don't buy the 18th-century pseudoscience of physiognomics -- the reading of people's characters from their features -- even though such ideas must have spurred on Houdon's art.

Whenever I read an account of a portrait's personality, I always find that I would have read the artwork in some very different way. The catalogue entry for Houdon's bust of a certain Jean-Sylvain Bailly, for instance, tells us that "thoughtfulness and intelligence emanate from his long, somber face." But I could have sworn that his thick, sensual lips and heavy-lidded eyes were the mark of an exhausted sybarite.

Houdon's Ben Franklin seems full of sparkling character. But I wouldn't want to bet on whether his careless grooming, lively features and barely parted lips are the mark of a calculating con man or of the frank plain-dealer he billed himself as. (Mouths are the other facial feature that Houdon lavishes attention on. This show lets us compare the actual life-mask he took of the Marquis de Lafayette, and the subtly different, more living bust that he derives from it, now with its mouth open just a hair. A wonderfully vivacious bust of Mme. Houdon shows her with a full and open smile; it gives a striking view of teeth in an age when such a broad grin was considered unbecoming.)

Jefferson's bust seems the spitting image of a Virginia gentleman of unrivaled mental acumen -- once you know it's him. Without that knowledge, I wouldn't be too sure that someone might not read him as a cold and calculating schemer. (Which, of course, he also was: He shrugged off Robespierre's Terror as a necessary bloodletting, then managed to fleece the French in one of history's great real estate deals, the Louisiana Purchase.)

I'll grant that Houdon's Louis XVI does seem the picture of a puffed-up sot -- though that could be because this exhibition sticks the petty tyrant out in front of a full spread of celebrated geniuses and leaders. Who wouldn't come off as a bit weak-willed alongside famous busts of Washington, Franklin, Jefferson, Lafayette and John Paul Jones?

But maybe it's a good thing that we can't settle on the sculpted monarch's character, any more than you can come to firm conclusions about your neighbor's personality. In Houdon's bust, we see life in the king's eyes, and that's what leads us to try to figure out what makes him tick. In a lesser royal portrait by a lesser artist -- whether working in clay or paint -- we might not be tempted to read much of any character into the inert heap of flesh it shows.

Houdon figured out how to use the painter's trick of making an eye twinkle, and so imbue his portrait busts -- from top, Thomas Jefferson, young Alexandre Brongniart and the singer Sophie Arnould -- with the illusion of personality.From left, Thomas Jefferson, Benjamin Franklin and Houdon's wife, Marie-Ange-Cecile. To make his portraits stunningly true to life, Houdon sometimes molded masks direct from his sitters' faces and even measured his patrons with calipers.Houdon studied cadavers before producing "L'Ecorche{acute}," a work still used to teach anatomy to artists.