You've all heard the one about the man who saved 3 million feet of string, or the woman who left all her money to her cat . . .

Certainly eccentricity is most usually considered to be its own reward. But there are occasions when we all become beneficiaries. The Mercer Museum complex in Doylestown in Bucks County, Pa., is one of those moments.

That Dr. Henry Chapman Mercer, archeologist and founder of the Moravian Pottery and Tile Works, was eccentric would be difficult to disprove; but that he left behind a unique and fascinating monument to his life and work cannot be disputed.

His singular efforts between 1908 and 1916 resulted in the construction of three amazing buildings: Fonthill, his home; the adjacent tile factory, after a fire destroyed an earlier wooden building; and the Mercer Museum. They are all stunning and bewildering. Indeed, bewilderment may be your initial response--and perhaps your most lasting impression.

The surprise begins at Fonthill where Mercer's imagination first took flight. He had no plans. He fashioned his home of reinforced concrete (then an untested technology) with the help of day laborers and a long-suffering horse named Lucy. If something didn't seem quite right, he'd have it torn out and they would start anew. And, on nearly every available surface of the interior are thousands of tiles.

What emerged was a bizarre fortress--a tribute to his faith in concrete, in his tiles and in himself. The imposing structure, set on a rolling meadow at the edge of Doylestown, is a celebration of tile: from ancient Delft and Portuguese tiles to Spanish and Arabic tiles; from mosaics and figured tiles to the Mercer tiles; tiles telling tales across whole rooms--all manner, shapes and colors of tiles. Overwhelming tiles. The vaulted ceilings are adorned; the walls, the halls--all abound in tile. No two of the 20 rooms are alike and even some of the furniture is made of concrete and, of course, covered in tile.

It is a fantasy world and you wonder at the man who created it.

Mercer, born in 1856, was successful with his tile business, but without the indulgence of his "patron" Aunt Laura, his archeological and acquisitive travels would not have been possible. He collected a staggering number of tools and artifacts. There are about 40,000 at the museum, and at Fonthill Mercer spread his prints and engravings throughout; rare old books fill countless concrete book shelves and spill over into wooden ones.

By design every room has at least two exits and every bedroom has its own bath. There is no dining room, but we can only speculate as to the reason.

The museum, with its awesome collection, is dedicated to preserving life in America prior to the Industrial Revolution. It was conceived from the inside out by Mercer as he envisioned suitable spaces to display his objects.

All this was intended for the public to appreciate, and upon Mercer's death in 1930 the museum passed directly to the Bucks County Historical Society (which he helped found in the 1880s). Fonthill, also operated by the society, has a separately administered trusteeship that included housing for his housekeeper until her death in 1975.

Exhibits and collections have remained exactly as Mercer arranged them and tours are guided by able and knowledgable volunteers.

Today, from 10 a.m. to 5 p.m., craftsmen will demonstrate authentic crafts, there will be food, games and a flower show and in a "sheep to shawl" event a wool shawl will be made from scratch in one afternoon starting with the sheep--all part of a crafts fair that reflects Mercer's concern for the pre-industrial age.

The three buildings are open from March 1--Dec. 31. The museum's hours are Mon.--Sat. 10 a.m.--4:30 p.m.; Sun. 1 p.m.--4:30, with self-guided tours.

Fonthill is open Tues.--Sun. 10 a.m.--3:30 p.m., with guided tours only, reservations requested.

The Moravian Pottery and Tile Works' hours are Wed.--Sun. 10 a.m.--4 p.m. Admission to the museum and Fonthill is $2; to the factory, $1.75. Special senior citizen, student and group rates are available.