The Calotype was a positive/negative process introduced in 1841 by Fox Talbot, and popular for the next ten years or so.
<Context ref.>A History of Photography, by Robert Leggat: CALOTYPE process, The [En línia]. [Consulta: 24 juliol 2005]. Disponible a: <http://www.rleggat.com/photohistory/history/calotype.htm>