On Friday, 2 May 1952, Rosalind Franklin and her doctoral student Raymond Gosling began a long X-ray exposure in the biophysics laboratories at King’s College London. The camera remained in operation until the following Tuesday, giving a total exposure of 62 hours. What emerged was Photograph 51, better known as Photo 51, the clearest diffraction pattern yet obtained from the highly hydrated B form of DNA.
The image did not show an individual DNA molecule, and the specimen was not a single strand. It came from a thin fibre containing hundreds of thousands of similarly aligned DNA molecules. According to King’s College London’s account of the experiment, that collective order is what allowed the molecular geometry to leave such a precise pattern on the film.
Sixty-two hours inside an X-ray camera
Franklin had arrived at King’s in 1951 with extensive experience studying carbon and other poorly ordered materials using X-ray diffraction. DNA presented a similar experimental challenge. Its fibres changed structure as their water content changed, producing a relatively dry and crystalline A form at lower humidity and a longer, wetter B form at higher humidity.
To control those conditions, hydrogen entering the camera was passed through salt solutions that maintained a chosen humidity. Filling the camera with hydrogen also reduced background scattering that could blur the resulting pattern. Franklin’s careful separation of the A and B forms was one of her most important contributions because it prevented measurements from two different molecular arrangements from being mixed together.
The DNA fibre used for Photo 51 was placed perpendicular to the X-ray beam. As X-rays struck the ordered atoms in the sample, they scattered and interfered with one another before reaching photographic film. The exposure had to continue for more than two days because the available source was weak and the narrow biological specimen scattered relatively little radiation.
What the dark cross revealed
An X-ray diffraction pattern is not a conventional photograph of the object being studied. Its spots and streaks encode repeating distances and symmetries within the specimen. Interpreting those marks requires calculations, measurements and an understanding of how different molecular shapes scatter radiation.
The most immediately striking feature of Photo 51 is the X-shaped arrangement of dark reflections. A helix viewed from the side produces just such a cross because its repeating turns scatter X-rays at regularly spaced angles. The pattern therefore offered unusually clear evidence that the B form of DNA was helical.
The distances between its layer lines supplied more information. They indicated a helical repeat of roughly 34 angstroms, with the stacked bases separated by about 3.4 angstroms. That meant there were approximately ten base units in each complete turn of the helix.
One missing reflection was especially informative. The absence of the fourth spot in each arm of the cross was consistent with two molecular chains displaced by three-eighths of a turn. Franklin’s wider analysis also placed the phosphate groups on the outside of the structure, with the bases directed inward.
Photo 51 was evidence, not a finished model
The photograph is sometimes described as if one glance at it disclosed every feature of the double helix. That compresses a much more complicated scientific process. Photo 51 strongly indicated a helix and yielded important dimensions, but determining the complete structure also required Franklin’s measurements of water content and density, chemical constraints, other diffraction photographs and physical model-building.
Franklin initially concentrated heavily on the more ordered A form because she believed its sharper reflections would permit a rigorous structure to be calculated directly from the data. The B form produced the dramatic helical cross, but it was less crystalline and therefore less suited to some of the mathematical methods she preferred. Her approach was to move cautiously from measurement to structure rather than begin with a speculative model.
By March 1953, however, Franklin had returned to the B-form evidence. Her draft analysis described a helical structure as highly probable and supported two chains, approximately ten bases per turn, inward-facing bases and outward-facing phosphate groups. She had independently extracted much of the essential geometry, even though she had not assembled the chemically complete base-paired model.
How the photograph reached Cambridge
Franklin and Maurice Wilkins worked in the same King’s research unit, but unclear divisions of responsibility and a deteriorating professional relationship kept them from collaborating effectively. When Franklin prepared to leave King’s for Birkbeck College, Gosling’s supervision and the DNA work were transferred back to Wilkins. Wilkins then had access to Photo 51.
In January 1953, he showed the image to James Watson during a visit to King’s. Franklin did not know that the photograph had been shown. Watson immediately recognised the clarity of its helical cross and carried the implications back to Francis Crick at the Cavendish Laboratory in Cambridge.
The photograph was not the only King’s information that reached Cambridge. Watson and Crick also became familiar with measurements and conclusions produced by Franklin and the wider King’s team. Those experimental constraints helped them reject impossible arrangements and build a model that was compatible with the known dimensions, water content and molecular density.
The three papers published together
Watson and Crick completed their correct double-helix model in early 1953. Their decisive chemical insight was complementary base pairing, with adenine paired with thymine and guanine paired with cytosine. That arrangement explained both the molecule’s dimensions and how genetic information could be copied when the two chains separated.
On 25 April 1953, Nature published three papers in sequence. Watson and Crick presented the model first, followed by a paper from Wilkins, Alec Stokes and Herbert Wilson, and then Franklin and Gosling’s paper containing Photo 51 and the experimental analysis behind it.
That ordering made the King’s work appear to confirm a structure that Cambridge had independently proposed. In reality, Photo 51 and Franklin’s other measurements had helped constrain and construct the model before publication. The distinction matters because supporting a completed theory is not the same contribution as supplying evidence used to create it.
The credit story is more complicated than one stolen image
The ethical issue remains real: Franklin’s unpublished information was used without her full knowledge, and Watson and Crick’s brief acknowledgement did not communicate how important the King’s data had been. The resulting public story elevated the Cambridge model-builders while leaving Franklin’s intellectual contribution difficult to see.
At the same time, historians increasingly resist portraying her as a scientist who merely took a useful photograph and failed to understand it. A 2023 reassessment in Nature drew on overlooked contemporary documents to describe Franklin as an equal participant in the effort. She was analysing DNA’s structure independently and understood the biological importance of the molecule rather than serving only as a technician for other researchers.
The episode was also the product of a dysfunctional research environment. Confused authority, weak communication, personality conflict and the exclusion experienced by women in the institution all shaped what happened. Reducing the history to either “Watson and Crick did everything” or “Photo 51 solved everything” replaces one distortion with another.
Franklin’s science extended far beyond DNA
Before joining King’s, Franklin had produced important work on coal, carbon and graphite. Her studies clarified why some forms of carbon can be transformed into graphite while others cannot, work with lasting relevance to materials science. She then strengthened her X-ray diffraction expertise in Paris before being recruited to investigate biological fibres in London.
Franklin left King’s in 1953 and established a productive research programme at Birkbeck College. As King’s biographical record notes, her later team investigated the structures of viruses. Her work on tobacco mosaic virus, turnip yellow mosaic virus and poliovirus helped establish the emerging field of structural virology.
This chronology is important because Franklin had not already solved the structure of tobacco mosaic virus when she made Photo 51. The virus research came afterward and demonstrated how much broader her scientific career was than the DNA dispute that later dominated her reputation.
The Nobel Prize and the lives that followed
Watson, Crick and Wilkins received the 1962 Nobel Prize in Physiology or Medicine for discoveries concerning the molecular structure of nucleic acids. Franklin was not among them. She had died from ovarian cancer in April 1958 at the age of 37, four years before the prize was awarded.
There is no firm evidence that her cancer was caused or accelerated by her laboratory X-ray work. Treating that proposed connection as established fact goes beyond the surviving medical and historical evidence. Her death should be reported accurately without turning an unresolved possibility into another dramatic certainty.
Watson lived until 6 November 2025 and died at the age of 97, according to Chemical & Engineering News. By then, institutional accounts of the discovery increasingly acknowledged that the correct model rested on experimental work by Franklin, Gosling, Wilkins and their colleagues at King’s.
The original plate and Franklin’s modern legacy
The original quarter-plate associated with Photo 51 remains in the archives of King’s College London. A digitised version and detailed catalogue record are available through the Wellcome Collection. An image once confined to a specialist laboratory can now be examined almost instantly from anywhere in the world.
Franklin’s name also appears on institutions and projects that extend far beyond historical commemoration. The Rosalind Franklin Institute, launched in 2018, develops technologies at the intersection of physical science, engineering and biology. Its emphasis on advanced imaging directly reflects the experimental tradition in which Franklin worked.
ESA’s Rosalind Franklin rover is currently scheduled for launch in late 2028. The ExoMars mission is designed to drill beneath the Martian surface and search ancient sedimentary material for possible signs of past or present life. A scientist once known chiefly through a contested photograph will have her name carried to another planet by a machine built to examine hidden structures.
A cross on a piece of glass
Photo 51 did not show the double helix in the familiar form found in textbooks. It showed a pattern of scattered X-rays produced by an ordered fibre, recorded slowly on photographic material over 62 hours. Turning that cross into molecular knowledge required Franklin and Gosling’s experimental skill, Franklin’s calculations and the contributions of several other scientists.
Its power lies partly in how much information was compressed into so little space. The dark arms disclosed a helix, their spacing disclosed its repeating dimensions, and an absent reflection helped disclose the relationship between two chains. The image was not the whole discovery, but the correct model could not be separated from the evidence it supplied.
More than seven decades later, the cross remains dark and the centre remains pale. What has changed is the frame around it, which now has room for the people, measurements and complicated exchange of information that made the double helix visible.
