On 2 May 1952, Rosalind Franklin and her PhD student Raymond Gosling set an X-ray camera running in a basement laboratory at King’s College London. The high-humidity DNA sample remained under the beam for a total of 62 hours before the plate was developed on 6 May, according to King’s College London’s account of Photograph 51. The result was the clearest image they had obtained of the wet, or B-form, of DNA.
The exposed plate carried a striking X-shaped arrangement of reflections. To a crystallographer, that pattern strongly suggested a helix, while the spacing of the reflections supplied measurements that helped constrain its dimensions.
It did not, by itself, reveal every part of DNA’s final structure. Watson and Crick had also already attempted an incorrect three-chain model, so Photograph 51 came before their first correct model, not before they had considered or constructed any model at all.
The 62-hour exposure
The sample was not a single molecular strand. Gosling later recalled winding 35 fibres around a modified paperclip because one fibre would have produced a pattern too faint to see, as recorded in Nature’s history of the other two DNA papers. Each fibre contained large numbers of DNA molecules aligned in roughly the same direction.
Humidity was crucial because DNA produced two noticeably different diffraction patterns. At lower humidity it formed the more ordered A-form, while additional water produced the elongated B-form represented by Photograph 51.
Franklin brought unusual technical control to the experiment. Her earlier work in Paris had trained her in X-ray diffraction, and at King’s she developed methods for controlling humidity, preparing fibres and obtaining interpretable patterns from difficult biological material.
What the cross meant
A helix viewed from the side contains repeating diagonal features. When X-rays scatter from that repeating structure, the resulting reflections form the characteristic cross visible in Photograph 51.
The vertical spacing of the spots indicated that the helix repeated after roughly ten stacked bases. However, the image alone did not reveal how many chains were present, which bases paired together or exactly how the molecule was chemically arranged, a limitation also noted in The Guardian’s reconstruction of the DNA work.
Franklin initially concentrated on the more orderly A-form because she believed it offered the better route to a rigorous crystallographic solution. She returned to the B-form evidence in early 1953 and drafted a manuscript in March describing a helical structure as highly probable, most likely with two chains, approximately ten bases per turn, bases toward the inside and phosphate groups toward the outside.
Who Raymond Gosling actually was
Photograph 51 was joint experimental work. Gosling prepared and mounted the fibres, operated the equipment and developed the plate while working under Franklin’s supervision as her doctoral student.
He had initially worked with Maurice Wilkins before being transferred to Franklin after she arrived at King’s. When Franklin prepared to leave for Birkbeck College, Gosling’s supervision returned to Wilkins, but the resulting Nature paper correctly carried the names of both Franklin and Gosling.
How the photograph reached Cambridge
As Franklin’s departure approached, Wilkins gained access to the B-form photograph and showed it to James Watson during a January 1953 visit to King’s. Franklin was not told that Watson had seen it, although later historical work has complicated the familiar account of a simple act of scientific theft.
A separate MRC report containing measurements from the King’s work also circulated within the research council’s network and reached Cambridge. Watson and Crick used some of those measurements as checks while testing possible models, a history discussed in Science News’ examination of the surviving archival documents.
With Linus Pauling also pursuing the problem, Sir Lawrence Bragg allowed Watson and Crick to resume model-building. They combined the King’s diffraction evidence with chemical constraints, Erwin Chargaff’s base ratios and repeated physical-model experiments until the pieces fitted.
What Franklin had worked out
The older popular story cast Franklin as a technician who produced an important image without understanding it. A more recent archival reassessment by historians Matthew Cobb and Nathaniel Comfort instead presents her as an active and informed participant in the solution, with overlooked documents suggesting that the King’s and Cambridge groups saw their work as more collaborative than Watson’s memoir later implied. Their argument appeared in Nature’s 2023 reassessment of Franklin’s contribution.
Franklin’s work established critical dimensions and provided powerful evidence for a helical structure. The complementary A-T and C-G pairing arrangement, together with the copying mechanism it suggested, emerged from Watson and Crick’s model-building using Chargaff’s ratios and other chemical information, as outlined by the National Human Genome Research Institute.
It therefore goes too far to say that Photograph 51 itself contained base pairing, the genetic code or the complete replication mechanism. Its achievement was narrower but still decisive: it gave exceptionally clear experimental evidence that any successful model had to explain.
The papers the world saw
On 25 April 1953, Nature published three consecutive papers on DNA. Watson and Crick’s model appeared first, followed by a paper from Wilkins, Alec Stokes and Herbert Wilson, and then Franklin and Gosling’s crystallographic paper, which included the B-form diffraction evidence associated with Photograph 51.
The Watson and Crick paper acknowledged that the authors had been stimulated by knowledge of the general nature of the King’s group’s unpublished results and ideas. It did not specify Franklin’s contribution or explain precisely how the unpublished information had reached Cambridge.
By publication day, Franklin had moved to Birkbeck College and was turning her attention to viruses, including tobacco mosaic virus. Her later work on viral structure became a major scientific contribution in its own right.
The Nobel and the missing name
Franklin died from ovarian cancer in April 1958 at the age of 37. Four years later, the 1962 Nobel Prize in Physiology or Medicine went to Watson, Crick and Wilkins for discoveries concerning the molecular structure of nucleic acids and their role in transferring biological information.
Her death meant that she was not among the recipients, but it is misleading to claim that a three-person limit automatically made her exclusion inevitable. The prize cannot tell historians how the committee might have evaluated the contributions had Franklin still been alive.
Watson’s 1968 memoir, The Double Helix, used the unwanted nickname “Rosy” and portrayed Franklin in terms that later scholars and colleagues strongly disputed. Cold Spring Harbor Laboratory relieved Watson of his administrative duties in 2007 and revoked his remaining honorary titles in 2019 after he repeated scientifically unsupported claims about race and intelligence. Watson died on 6 November 2025 at the age of 97.
Why the photograph still matters
Photograph 51 was one of several pieces of evidence that converged on the double helix. The National Human Genome Research Institute credits Franklin, Gosling’s experimental work, Wilkins, Watson and Crick within the wider process of using diffraction patterns and physical models to establish DNA’s structure.
It belongs to a broader history of scientific images that made difficult arguments visible. Robert Hooke’s drawings of cork cells and John Snow’s map of cholera deaths served different purposes, but each turned an otherwise abstract pattern into something a wider audience could examine.
The double helix later became foundational to molecular biology, genome sequencing and biotechnology. Those developments did not descend from Photograph 51 alone, but the image helped supply the experimental geometry on which the successful structure depended.
What the basement produced
The experiment required a bundle of carefully aligned fibres, controlled humidity, a narrow X-ray beam and more than two and a half days of exposure. What emerged was a small photographic plate whose cross sharply narrowed the range of structures that DNA could possess.
The image did not solve the molecule in isolation, and it did not eliminate the need for chemistry, mathematics or model-building. It did something equally important: it forced every serious model to account for a clear and measurable helical pattern.
That is why Photograph 51 remains one of science’s most recognisable experimental images. Its importance lies not in a myth that one photograph contained the whole future of genetics, but in the precision with which Franklin and Gosling made part of an invisible molecule visible.
