“Dictyota dichotoma, in the young state; and in fruit,” a cyanotype by Anna Atkins, c. 1850, from her book Photographs of British Algae: Cyanotype Impressions, Part XI. Atkins’s work is currently on view in the exhibition Blue Prints: The Pioneering Photographs of Anna Atkins, at the New York Public Library, in New York City.

Courtesy The New York Public Library, Astor, Lenox and Tilden Foundations