Anna Atkins laid each specimen of seaweed flat upon paper soaked in iron salts, placed a sheet of glass over it, and let the sun print its shadow.
The process — cyanotype, invented in 1842 by her friend Sir John Herschel — renders everything it touches in two notes: Prussian blue, and the white ghost of the plant. There was no negative, no enlargement, no darkroom; only sun, water, and the algae themselves, pressed to paper one by one.
Between October 1843 and 1853 she produced a complete photographic flora of the British algae in three volumes, with hand-lettered titles printed by the same cyanotype process. About a dozen sets survive. This page reproduces the New York Public Library’s copy — Sir John Herschel’s own — and pairs each plate with the species it depicts, its habitat, the words handwritten on the print, and, where possible, a modern photograph of the same alga taken in the wild.