A cyanotype of Hymenophyllum decurrens, or Sierra de Luquillo Filmy fern, from 'Cyanotypes of British and Foreign Ferns', made by Anna Atkins in 1853.
Atkins was a pioneering figure in photographic history, having produced the first book to use photographic illustrations - 'British Algae: Cyanotype Impressions' - in 1843.
This image was made by placing the plant specimen on top of light-sensitised paper and then exposing it to sunlight.
The cyanotype process was invented by Sir John Herschel (1792-1871) in 1842 and derived from his observations on the light sensitivity of iron salts. The brilliant blue colour of the resulting prints gives the process its more common name - the blueprint. The process was used for many years to duplicate engineers' drawings.