Among the 50,000 who filed through 291 was an art student named Georgia O'Keeffe. She would later be the subject of Stieglitz photos taken, she said, with "a kind of heat." Still later they married. But in 1908, she was most impressed by his passion for art. "I very well remember the fantastic violence of Stieglitz's defense when the students began talking with him about the drawings. I had never heard anything like it."

Today, smart phones make photography simpler than a snap. More photos are now taken each day than were taken in the 19th century. Images are everywhere, and nowhere.

But imagine. . . It's 1905, a snowy day. You take the rickety elevator. You walk down the hall. You see the photographs. You look at them, look, look. . . And when you descend to the street, you see them again. There is the snow. There is the light, the beauty, the "ultimate truth," just as Alfred Stieglitz and friends captured it in photographs, in art, at 291.