Between May 2 and May 6, 1952 English molecular biologist Rosalind Franklin, working at King's College, Cambridge took photograph No. 51 of the B-form of crystalline DNA. This was her finest photograph of the substance, showing the characteristic X-shaped "Maltese cross" clearer than before.
About eight months later, on January 26, 1953, Franklin showed this photograph to physicist and molecular biologist Maurice Wilkins. Four days later, on January 30, 1953 Wilkins showed the photograph to James Watson.
The following day Watson asked laboratory director Lawrence Bragg if he could order model components from the Cavendish Laboratory machine shop. Bragg agreed. Watson's account of Franklin's photo 51 to Francis Crick confirmed that they had the vital statistics to build a B-form model: the photo confirmed the 20Å diameter, with a 3.4Å distance between bases. This, plus the repeat distance of 34Å, helix slope about 40°, and the likehood of 2 chains, not 3, seemed to be sufficient to build a model.
Franklin's file copy of Photograph 51, labeled in her handwriting, is preserved at the J. Craig Venter Institute.