(Niépce used the term 'heliographic' and Talbot used the term 'photogenic')
In 1840 Herschel describes how paper that is soaked in a solution of silvernitrate, silverchloride and silverbrmide reacts in sunlight. He discovers that silverbromide is much more light-sensitive than the other silver salts.
Herschel published the results in 'On the Chemical Action of the Rays of the Solar Spectrum on Preparations of Silver and other Substances, both Metallic and Nonmetallic, and on some Photographic Processes.'
Already in 1840 Herschel mentioned that "we must create a new photography based on silverbromide".
In another publication in 1842 'On the Action of the Rays of the Solar Spectrum on Vegetable Colours and on Some New Photographic Processes' Herschel describes for the first time the iron process with ammoniumcitrate. He describes the process with blue lines on a white background and the process with white lines on blue background (the so-called blueprint). Herschel also discoverd the chrysotype process that uses the exposure of iron salts followed by a development with gold-and silver solutions.
