Rosalind Franklin, whose photographs of the molecular structure of DNA led to a 1962 Nobel Prize for three other scientists, knew that her famous images held the secret of the double helix, new research suggests. An examination of archival material reveals that Franklin's Photograph 51, which was taken in May 1952 and later published to show the double-helix structure, was specifically taken for the purpose of publication — not, as some narratives have held, an experimental shot that Franklin believed was insignificant. The new research revealed that Photograph 51 was a new exposure of a sample Franklin had already photographed. She started Photograph 51 the same day she saw that photograph,...