This book is a catalogue of an exhibition held at the National Gallery of Art, the Art Institute of Chicago, and the Los Angeles County Museum of Art. At this sesquicentennial celebration, it has been our intention in both the exhibition and the catalogue to present and analyze those photographs that, regardless of why they were made, seem the most visually significant. Limiting ourselves, for the most part, to American and European photographers, we have attempted to chart the development of an understanding of photography as a pictorial device. - Introduction.