In this photograph, a stratus cloud, caught like a burst of light against a dark surface, animates the image with a sense of excitement, awe, and perhaps even revelation. The pho-tographer, Alfred Stieglitz, began to photograph clouds in 1922, tilting his camera toward the sky to produce dizzying, nearly abstract images of their fleeting configurations. Over the next decade Stieglitz produced some 350 cloud studies, which he called Equivalents. For Stieglitz, the series was both a technical and an aesthetic challenge. He maintained that these cloud photographs did not just fix ever-changing forms but also expressed pure emotion, even attributing to them the evocative power of music.