The famous X-ray image of DNA associated with Rosalind Franklin is called Photo 51. It was a diffraction photograph of DNA's B form and became crucial evidence in identifying the molecule's helical structure.
Photo 51 was made in May 1952 at King's College London. Although Franklin supervised the work and is strongly associated with the image, it was actually taken by her doctoral student Raymond Gosling. The number 51 referred to the photograph's place in Gosling's series of diffraction images, not to a year or a chemical measurement.
The image showed a distinctive X-shaped pattern, a visual clue produced by a helix. Maurice Wilkins later showed it to James Watson, who was working with Francis Crick on a model of DNA at Cambridge. Together with other experimental data, the image helped them develop the double-helix model published in 1953.
Photo 51 is sometimes incorrectly described as a conventional photograph or solely as Franklin's own image. More precisely, it was an X-ray fiber-diffraction image taken under her supervision. Franklin's broader experimental analysis was also essential to understanding DNA's structure.