From Wikipedia
Photo 51 is the name given to an X-ray diffraction image of DNA taken by Rosalind Franklin in 1952 that was critical evidence in identifying the structure of DNA. The photo was taken by Franklin while working at King's College London in Sir John Randall's group.
The photo, shown by Maurice Wilkins to James D. Watson, was the critical evidence that led to the confirmation of the postulated double helical structure of DNA, published in a series of five articles in the 1953 issues of the journal Nature. Franklin and Raymond Gosling's own publication in the same issue of Nature was the first publications of this more clarified X-ray image of DNA.