Which molecule's double-helix X-ray image was Photo 51?

Photo 51 is a single X-ray diffraction photograph of the hydrated B form of DNA, exposed over roughly sixty hours in May 1952 at King's College London. It was taken by Raymond Gosling, a doctoral student, working under Rosalind Franklin, whose exacting control of the humidity around the fibre is what made the pattern clean enough to read.

To someone who could read it, the image gives the structure away. The blunt X across the middle is the signature of a helix. The spacing of the layer lines yields a helical repeat of about 34 angstroms; the heavy smear at the top and bottom gives 3.4 angstroms, the distance between stacked bases. The fourth layer line is missing, which says the two strands are not evenly spaced but offset - the major and minor grooves.

In January 1953 Maurice Wilkins showed the photograph to James Watson without Franklin's knowledge. Watson later wrote that his mouth fell open and his pulse raced. The model followed within weeks. Franklin died of ovarian cancer in 1958, aged thirty-seven; the Nobel went to Watson, Crick and Wilkins in 1962, and is not awarded posthumously. The argument about what she was owed has run ever since, and the photograph is the reason it is not merely sentimental.