"The cyanotype process uses a mixture of iron compounds, which when exposed to UV light and washed in water oxidize to create Prussian Blue images.  The technique was invented in 1841 by Sir John Herschel and was popularized by photographer and botanist Anna Atkins.  Her book Photographs of British Algae: Cyanotype Impressions, published in October 1843, is considered the first photographically illustrated book."  (Parallax Photographic Coop)