The 1806 directive from President Thomas Jefferson sent two young Army captains on a two-year, 6,000 mile journey to explore the new Louisiana Territory.

“The object of your mission is explore the Missouri River and such principal stream of it as by its course and communication may offer the most direct and practicable water communication across the continent,” he wrote. They never found a northwest passage, but their journey is part of American history, the Lewis and Clark Expedition.

Two hundred years later, a physicist-turned-photographer named Greg MacGregor set out to retrace the journey of Meriwether Lewis and William Clark, and his work comprises a new show at the Napa Valley Museum, “Lewis and Clark Revisited: A Photographer’s Trail.”

Using the journals of Lewis and Clark as his guide, MacGregor followed their trail along the Missouri River, up through South and North Dakota, across Montana and finally to the Pacific Coast. His project, done in stages, took six years and ultimately covered 16,000 miles. “For inspiration before each outing, I would read my favorite passage from the journals,” MacGregor wrote in an introduction to the show.

His images are paired with excerpts from the journals, creating a striking contrast of the landscape of America, then and now.

Rather than striving to recreate the pristine lands that Lewis and Clark explored, MacGregor’s black and white images document the world that the white man built as he moved west.  Dams, towns and bridges fill the modern trails. MacGregor wrote that although he wanted to capture “a sense of the grand nature they traversed,” he didn’t try to avoid the current condition — the industrial section of Billings, Mont., or the gambling casino on the Missouri River in Council Bluffs, Iowa; the plastic buffalo and signs for 5 cent coffee in South Dakota, and even the tourists snapping photos of William Clark’s name, where he carved it into a rock at Pompey’s Pillar, Mont.

Nonetheless, much of the grand landscape remains, captured in MacGregor’s photos, creating at once a compelling look at the American west, a dramatic illustration of a 200-year old adventure and the world that has grown up in the explorers’ trail.

“Lewis and Clark Revisited” will be at the Napa Valley Museum through Jan. 7. The museum, which is on the grounds of the Veterans Home of California in Yountville, is open every day except Tuesday, from 10 a.m. to 5 p.m. Admission is $4.50 for adults, $3.50 for students and seniors, $2.50 for youth 7 to 17, and free for children under 7. For additional information, call 944-0500, ext. 101.