Assembled by four generations of Marquesses of Hertford and given to the nation by the widow of Sir Richard Wallace, illegitimate son of the fourth, this collection of art and artifacts is important, exciting, undervisited -- and free. As at the Frick Collection in New York, Hertford House itself is part of the show: the fine late-18th-century mansion, built for the Duke of Manchester, contains a basement floor with educational activities, several galleries, and a courtyard, covered by a glass roof, with exhibit space and an upscale restaurant.

The first marquess was a patron of Sir Joshua Reynolds, the second bought Hertford House, the third -- a flamboyant socialite -- favored Sèvres porcelain and 17th-century Dutch painting; but it was the eccentric fourth marquess who, from his self-imposed exile in Paris, really built the collection, snapping up Bouchers, Fragonards, Watteaus, and Lancrets for a song (the French Revolution having rendered them dangerously unfashionable), augmenting these with furniture and sculpture and sending his son Richard out to do the deals. With 30 years of practice behind him, Richard Wallace continued acquiring treasures after his father's death, scouring Italy for majolica and Renaissance gold, then moving most of it to London. Look for Rembrandt's portrait of his son, the Rubens landscape, Gainsborough and Romney portraits, the Van Dycks and Canalettos, the French rooms, and of course the porcelain. The highlight is Fragonard's The Swing, which conjures up the 18th-century's let-them-eat-cake frivolity better than any other painting around. Don't forget to smile back at Frans Hals's Laughing Cavalier in the Big Gallery or pay your respects to Thomas Sully's enchanting Queen Victoria, which resides in a rouge-pink salon (just to the right of the main entrance). There is a fine collection of armor (which you can try on for size) and weaponry in the basement as a break from all the upstairs gentility. COST: Free. Tube: Bond St.