Don't let the cliche' about "a vanishing frontier" stop you from seeing the information-packed new show at the Museum of Natural History.

Because what you find, as you roam through room after room of sketches, watercolors, engravings, stuffed animals, Indian moccasins, tomahawks and leggings, is nothing less than the lost world of America as it looked 150 years ago, an uncluttered world of open fields and unending vistas -- even in the populous East.

More haunting still are the portraits of the western Indians -- Mandans, Blackfeet, Sioux -- in their heyday. The vintage photographs of Indian life in our archives were taken toward the end, when this hunting-farming culture in our midst had been all but destroyed. But the paintings of Swiss artist Karl Bodmer and the journals of Prince Maximilian of Wied, a German naturalist, catch the Indians in their prime, before the westward invasion overwhelmed them.

"Views of a Vanishing Frontier" contains more than 100 works by Bodmer and other relics from their 1832-34 expedition, which retraced the path of Lewis and Clark along the upper Missouri River to Great Falls, Mont. The show remains up through March 31.

Luckily for us, Bodmer didn't wait until he got out West to start drawing. A charmingly pastoral watercolor shows "Bethlehem on the Lehigh": a few houses and a church on a turnpike amid wheat fields and towering elms. It is, of course, Bethlehem, Pa., but it could easily pass for, say, Old Deerfield Village in the Pocumtuck Valley of Massachusetts.

A meticulous watercolor of New Harmony, Ind., with its stake fences, its church building, its clotheslines hung with laundry, its fine broad spaces, gives you a sense not only of what the country must have looked like then but of how it felt to be alive in those unhurried days: Horsemen amble past, people stop to chat in an open yard, dogs cavort in their stiff-legged ritual of encounter.

You know from these preliminary pictures that you are in the hands of an accurate observer and skilled artist. It helps when you see the later sketches of unbelievably fantastic rock formations on the Missouri and the romantic apparition of the Indians themselves.

Only the work of George Catlin, who visited the same tribes a year earlier and left a magnificent record of Plains life (including portraits of some of the same chiefs), approaches these scenes in their feeling of authenticity, their sense that it must really have looked this way. Catlin and Bodmer didn't know how fortunate they were in their timing: The Blackfeet had just turned more or less friendly after decades of hostility to whites; three years after the expeditions left, the tribes of the upper Missouri were decimated by smallpox, imported on a fur company steamboat.

The Indians were still wild then. They painted and tattooed their bodies in the full belief that the ceremonial marks had power in them. They made pictures of the great events in their lives to preserve the special magic of what had happened.

Even a trade item such as a metal tomahawk head with a peace pipe built into the heel has been given character and maybe a touch of magic. The one on exhibit here has bits of mirror embedded in the shaft.

One Mandan warrior, Mato Tope, won an epic hand-to-hand fight with a Cheyenne chief by grabbing his opponent's knife, jerking it from him and killing him with it. Again and again he pictured the coup on war shirts and borrowed paper, showing the blood pouring from his slashed palm. For the rest of his life he wore a wooden knife in his hair as a talisman.

His paintings and several other stylish drawings by Indians are part of the show.

One remarkable Bodmer engraving captures an Indian skirmish witnessed by the travelers at Fort McKenzie, near Great Falls. Thinking the fort itself was being attacked, Prince Maximilian's party rushed up to the palisade and saw some 600 Crees and Assiniboines charging the 20 lodges of Piegans just outside. The fighting went on all day, and Bodmer's version of it has been called by Smithsonian ethnologist John C. Ewers "the most accurate and dramatic portrayal of an intertribal battle by a white artist who had witnessed the action." A wall-size blowup enhances the drama.

A detailed report of this incident and of many more adventures along the way is featured in an essay by Ewers in the show's catalogue.

When Maximilian returned to Germany he published his journals in two volumes that surely stimulated -- if they didn't actually touch off -- the traditional German fascination with the American Indian. His illustrated manuscripts and Bodmer's 427 drawings were stored in the prince's Rhine castle, lost to the world until after World War II.

At that point a German researcher discovered the papers in the Wied family library. He is probably still talking about it.

In 1959 Maximilian's heirs sold the collection to a New York art dealer, and three years later it was bought by Northern Natural Gas, now InterNorth Inc. This show, sponsored by InterNorth, opened last year at the Joslyn Art Museum in Omaha and went on to Fort Worth and San Francisco before winding up here.

Quite aside from the art, the exhibit tells a lot about travel in the American West in 1833. There are frighteningly vague maps of Indian territories, a copy of Lewis and Clark's own map of the Missouri (Clark gave it to the voyagers when he took a fancy to them in St. Louis; the original was later lost, making the copy enormously valuable), and hotel bills for one Baron Braunsberg.

That was Maximilian. Titled figures often toured America incognito in those days. A prince never could tell what sort of people he might run into over here.