Daguerreotype

Image formed on a silver-coated copper plate, sensitized by fumes of iodine. The image is developed in mercury vapor, which produces a unique direct positive image. Announced in 1839 by Louis-Jacques-Mandé Daguerre, who had developed this process after his partnership with Joseph Nicéphore Niépce. Popular until 1860.

Source: http://www.ifpda.org/content/collecting_pr...