Direct positive photographic processes produce unique images, captured without a negative. Various direct positive processes are represented in this selection of 19th-century portrait photographs, including daguerreotypes, ambrotypes, ferrotypes or tintypes, and salted paper prints.

Johnson, Melissa and Ted Stanley. “PRESERVING PRINCETON’S CASED IMAGES.” The Princeton University Library Chronicle 62, no. 2 (2001): 276–86.

Rothrock, O. J. “NINETEENTH-CENTURY PHOTOGRAPHY IN THE LIBRARY.” The Princeton University Library Chronicle 39, no. 1 (1977): 30–37.

Thierry, Mary. “MIRROR, MIRROR: AMERICAN DAGUERREAN PORTRAITS.” The Princeton University Library Chronicle 73, no. 3 (2012): 421–31.

(by Julie Mellby unless otherwise noted)