The great power in old photographs is the things going on around the edges: the true life of a lost time, all unaware that it is being stared at by the future.

The hundred pictures in the Smithsonian's new "Photographing the Frontier" exhibit have this quality. The show, opening this week, will run in two parts, to Oct. 23 and then later in the winter.It is on the third floor of the Museum of History and Technology.

Some men pose self-consciously on a delivery wagon, but what we look at is David Eccles' general store in the background with its plug tobacco ads and grim hominess. An Arizona hermit glares out from behind his fright-wig beard, looking like Rip Van Winkle 20 years after, but what catches the eye amid all the rags and tatters is the rifle he holds in his permanently dirty hands: sleek and oiled and well used.

There are school yards here, and stores (why do butcher shops always have gaudy nature paintings high on their back wall?), and hangings, and deputies armed for a war, it looks like, and gold miners in Oregon. There is a strange man wearing a suit covered with peanuts. There are funerals with the body propped up in the middle of the relatives ("He looks so natural - hasn't looked that good in years . . .") and portraits of the photographers themselve, as awkward as any of their subjects ever were.

We have Eugene Ostroff, curator of photography, to thank for this fascinating documentary of American life in the Northwest and Southwest in the last half of the 19th century. Avoiding the merely photogenic (though there is a magnificient and strange panorama of a valley under the buttes, with a single lonely mansion and off in a field a tiny speck that proves to be a horse and carriage), Ostroff gives us the frontier life as we rarely see it in the movies.

Not everybody in the West was a gunslinger. Not everybody galloped across the scenery toward some theatrical confrontation. You could live your whole life and never see a gunfight.

"We never paid much attention to all them guys," one oldtimer was quoted recently. "Just a bunch of roughnecks. Crazy kids. We just went on about our business."

He was talking about Dodge City.

Another show just opened is Pyramid Gallery's exhibition of Roy Slade's work. Slade, former director of the Corcoran Gallery, this year was named president of Cranbrook Academy of Art.

He is represented by some large acrylis, some tentative drawings and, perhaps most striking, several crayon studies of vivid suns and moons. These are done in parallel strokes as though the forearm pivoted from the elbow and gradually covered the paper with arcs of color. The effect is vibrant with concentrated energy.

By contrast, the acrylics, while done in the same bright reds and blues, have a dreamlike flow, an underwater feeling.

Notable at the Hirshhorn are a retrospective of Thomas Eakins and photos by Leland Rice in the museum's first loan exhibition of contemporary photography.

The Eakins show begins with a remarkable miniature done at age 9, chronicles his four years of study in Paris, where he was much influenced by Rembrandt and others, and documents his abiding fascination with horses, anatomy and perspective problems.

Some 1884 photos, Muybridge-like studies of a nude man jumping, apparently led Eakins into photography himself. He used these pictures as studies for his paintings, but today it is the formal oil portraits that seem-pedestrial and the photos and sketches that arrest the eye. One exception is the eerie unfinished last portraits: a clerick, his head and shoulders fairly well fleshed out, almost complete. But no eyes.

The 35 photos by Rice, a Californian, include his chair pictures, some studies of walls and doorways, and the later work with color, in which the dramatic contrasts of the earlier years are softened.