HE FIRST THING that happens when you visit the new exhibit at the National Geographic Society is that you bump your nose against a display case. The next thing is, you bump your nose again, and maybe your glasses fall off.

It goes on and on like that, because the exhibit is a fabulous assembly of miniatures made by modern American masters, and you just can't bend close enough to these tiny triumphs to satisfy the eye.

Most are on the standard dollhouse scale of 1:12, an inch to the foot, but they're executed in such detail and with such perfection that the mind denies their actual size. A needle case is the size of a grain of rice; in it are sewing needles. Threaded sewing needles.

There are thousands upon thousands of diminutive delights on display, each more marvelous than the last: racing cars complete with dings and tiny oil leaks; cut-glass punchbowls holding half a thimbleful; silver services you could set out on your thumbnail; needlepoint carpets with 3600 stitches per square inch; pickup sticks the size of splinters, each one turned, tapered, polished and painted.

What keeps the eye from wearying of this infinite fussiness is the imagination and wit of the modelers. Madelyn Cook's Art Deco dream, a breadbox-size room done in mirrors and black Plexiglas, seizes the attention with flash and gleam, but holds it with subtleties of proportion, texture and inspired unbalance. Tom Roberts' "Homage to Vermeer" is so telling an evocation of the Dutch master's domestic interiors that the mind tries to dismiss its third dimension.

Equally painterly, and bold as brass, is Tim Prythero's beat-up house trailer, a dead-solid-perfect rural slum done with the unblinking yet somehow affectionate realism of those WPA photographs from the Great Depression. From the roof patches held down by old tires to the doorscreen torn by heedless children, this diorama-style setpiece is flawlessly flawed. In the unseen kitchen, one knows, a faded woman in a faded cotton dress sits slumped at a wobbly table, with hardly enough energy to swat at the flies foraging over the sticky oilcloth. It is artifice raised to the power of art.

If you don't have all the time in the world it might be best to rush past all these wonders to the crowning exhibit, William R. Robertson's Georgian mansion, which is the sort of dollhouse Faberge' might have built for a Tsarina.

Robertson bristles at the term dollhouse, blanches at the idea of children playing with his creation. "This is for kids over 55," he grins. Too much attention to details of its construction risks tends to obscure the beauty of the result, but it's hard to resist some recitation:

The 13-room mansion contains about 75,000 handmade parts, including 12,000 walnut shingles, 500 feet of beaded siding, a hidden framework of more than 1,000 feet of aluminum bar stock, and random-width cherry flooring pinned with more than 3,000 pegs. The sash windows work, and have glass panes. The clocks work. The place is full of secret panels and secret passages, which Robertson delights to tell about, and secret secrets, which he keeps to himself.

The teeny-weeny doorknobs turn, operating teenier-weenier spring-loaded latches. Assuming of course that somebody hasn't locked the door and taken away the key.

It took Robertson, who became a professional miniaturist because his magnificent obsession leaves him no time for anything else, about 6,000 hours to build the house. That is the equivalent of more than two years of working eight hours a day, seven days a week, never a day off. And he had help: his mother, Esther Robertson, put some 700 hours into the painting alone, including the more than 10 dozen different types of interior hardwood moldings.

The house was commissioned by the Miniature Museum of Kansas City (Missouri), the major lender to the exhibit. How can Robertson bear the thought of letting go of the place after investing so much of his life in it?

Well, the satisfaction is as much in the conception and execution as in the results, he says: "It's very rewarding, very meditative work." And of course he builds these things for a living, knowing he'll have to let them go.

And anyway while he was at it he built another house just like this one, just for himself.

SMALL WONDERS -- Through January 1 in Explorers Hall, National Geographic Society, 17th and M NW. Open every day but Christmas; hours 9 to 5 Monday through Saturday, 10 to 5 Sunday. Free.