Sarah Greenough, senior curator of photographs, National Gallery of Art, and Roger Taylor, professor of photographic history at De Montfort University, Leicester. Two methods of fixing an image dominated the early days of photography: the one-of-a-kind daguerreotype and the replicable calotype, which was made using paper negatives. In the first of this two-part episode, Gallery curator Sarah Greenough, and Professor Roger Taylor of DeMontfort University discuss the emergence of the calotype and how it competed with the emergence of glass negatives. Produced in conjunction with the exhibition Impressed by Light: British Photographs from Paper Negatives, 1840-1860.