The key to understanding why Franklin took Photograph 51 lies in her notes about a previous picture. It was the forty-ninth in a series of seventy-eight diffraction images she and her graduate student Raymond Gosling captured with a Philips micro-camera (see Fig. 2) at King’s College London in 1951 and 1952.^{Footnote 4} Months before taking Photograph 49, the pair had established the existence of two distinct structural forms of DNA, and they had begun to manipulate samples intentionally to create them. The A form possessed an almost crystalline structure and gave diffraction patterns with many spots, potentially offering a great deal of information for structural analysis (by calculation of Patterson maps).^{Footnote 5} DNA could be converted to the B form at high relative humidity, yielding a simpler diffraction pattern with too few spots for similar (Patterson) structure calculations. Franklin had discussed her discovery of the distinct A and B forms in a colloquium at King’s on 21 November 1951 and documented them in an annual report of 7 February 1952, but her results were not otherwise published.^{Footnote 6}

Franklin’s annual report of February 1952 also laid out a research plan that she appears to have followed closely until she moved from King’s College to Birkbeck College in the spring of 1953. Her goals were twofold: (1) to determine what caused a given DNA sample to exhibit the A or the B form, and afterward, (2) to characterize both forms initially through Patterson analysis of the more complex but data-rich A pattern.^{Footnote 7}

Photograph 49 was created two months later in the course of a lengthy series of experiments aimed at completing the first of these objectives. Franklin’s stated hope was that these experiments would also reveal the best techniques for capturing sharp DNA diffraction patterns. As hoped, it was during this period of systematically manipulating the humidity levels at which samples were stored and photographed that she and Gosling produced what they retrospectively considered their best ever A form pattern (Photograph 42 of early February 1952) as well as their best B form photographs in May.

Completed on the morning of 2nd May 1952, Photograph 49 showed the result of aiming an extremely fine X-ray beam for three days and nights at a fiber that had been pulled into a fine thread from a drop of gel-like concentrated DNA solution.^{Footnote 8} The beam emerged from a collimator at the front of the camera and met the sample fiber, which was mounted and held in place across the orifice by two drops of glue, immediately inside the camera, just 15 mm in front of the X-ray film that recorded the diffraction pattern (see Fig. 2). Because space within the camera was so tight, they employed an unusual technique to prevent the main (undiffracted) X-ray beam from overexposing the center of the film. Instead of blocking the main beam with a small circular piece of heavy metal such as lead between the specimen and the film, as was conventional in larger cameras, they punched a hole through the centers of all the stacked films and allowed the undiffracted beam to pass right through the films and out of the back of the camera, through a small fluorescent screen that facilitated camera alignment (the holes can be seen in the film and camera body shown in Fig. 3).^{Footnote 9} Franklin and Gosling controlled the relative humidity of the DNA sample throughout the days-long experiment by passing hydrogen gas that had been bubbled through an aqueous salt solution of defined composition and concentration into the body of the camera.^{Footnote 10}

As with the trials that preceded Photograph 49, they would learn the result of the experiment by removing and developing a stack of two or three small, hand-cut pieces of film from where they were held inside the camera body during the exposure (see Fig. 3). Each of the duplicate (or triplicate) films would show an even smaller X-ray diffraction pattern about 2.5 centimeters in diameter. On this occasion, they had used two pieces of film marked in Franklin’s handwriting as 49A and 49B (see Fig. 4).^{Footnote 11}

These Photograph 49 films showed an unprecedentedly sharp version of the diffraction pattern produced by DNA’s B form (Fig. 2). Franklin and Gosling accomplished this by using a sample that had become irreversibly locked into the high-humidity B structure, meaning that it could be mounted taut in the camera and photographed at the lower 75% humidity level that ordinarily produced an A form pattern.^{Footnote 12} The resulting fine definition made it possible to use the photograph for more precise analysis than any previous B image had allowed, and Franklin therefore gave it her prompt attention. This immediacy is evident from a separate series of notes that were headed “(49) 49B Rough measurements on projection,” dated 2 May 1952 — the very same day she and Gosling had developed the films.^{Footnote 13} These notebook pages began with measurements of the new diffraction pattern, which the pair enlarged by projecting it onto a piece of white cardboard, and Franklin’s notes progressed in short order to making general conclusions about the B structure of DNA. She noted that the molecule showed repeats occurring every 34 Å, meaning it was about 25% longer per repeating unit than the drier, more crystalline A form she had already begun to analyze.

From the day Photograph 49 was taken, Franklin interpreted the pattern as indicative of a helix.^{Footnote 14} Other evidence indicates that she already had a helical interpretation of the B form structure in mind, and now she calculated how many layers of purine and pyrimidine bases there must be “per turn of helix (if there is a helix).”^{Footnote 15} If the bases were spaced 3.4 Å apart, as she and other researchers believed, then her calculations suggested that there would be exactly ten nucleotide-base layers in each lengthwise repeat of the molecule. In her notes, she phrased the finding more generally: “there is an integral number (or single fractional number) of residues per [34 Å] turn.”^{Footnote 16} Later, in 1954, Watson and Crick acknowledged that they had built their double helix as a model of the B form with its 34 Å axial period containing ten base layers per turn, which, Watson and Crick crucially realized, were complementary base pairs rather than individual bases. It is not widely appreciated that these parameters were originally established in Franklin’s analysis of Photograph 49.^{Footnote 17}

Today Photograph 49 survives in its entirety only as a negative-image contact plate of film 49B held at the King’s College London archive (Fig. 2). The photograph is remarkable for two distinct reasons. First, it vividly captures the very same diffraction pattern known so well today from Photograph 51. Secondly, however, a portion of the famous pattern has been cut off. Through our examination of the camera Franklin and Gosling used in 1952, we can explain how this must have happened. The photographic films were held in place by a flat metal bracket whose edges were bent around a backing plate that the films rested against (Fig. 3). This holder has a slightly irregular circle of roughly 2.5 cm diameter cut out to allow the diffracted X-rays to hit the films, and the glass negative of 49B illustrates that this holder had become misaligned with the X-ray beam and the DNA sample. In consequence, the upper part of the pattern is cropped to such an extent that a large, arc-like region now familiar from Photograph 51 (Fig. 1) does not appear in the image. This missing arc, like its symmetrical counterpart which appears at the bottom of Photograph 49, is caused by the 3.4 Å spacing between each layer of nucleotide bases in the molecule.

As we have seen, this inadvertent cropping did not prevent Franklin from using Photograph 49 in her analysis and calculations. She had captured enough of the pattern to be able to discern that one of the 3.4 Å arcs was located on the tenth meridional layer line of the pattern and to understand the implications for quantifying the layers of bases stacked in a single 34 Å turn of the helical molecule. Photograph 49 was a sharp and usable image but, because of the equipment malfunction, it was poorly composed and thus less than ideal for public presentation.

Franklin therefore decided to retake the photograph. On the evening of 2 May 1952 – still the same day she first saw Photograph 49 – she began the notebook entry for Photograph 51: she would use the same specimen from 49, the same X-ray setup, and the same 75% relative humidity within the body of the camera.^{Footnote 18} The only thing distinguishing this from being an exact repeat was, as she wrote, that Photograph 51 was taken “with [the] holder centered over [the] collimator so as to include both 3.4 [Angstrom] arcs.”^{Footnote 19}

Photograph 49 had been a research image, the result of an experiment. Photograph 51 would be the exact opposite: a refined image taken after the fact to document that experiment’s particularly successful outcome.^{Footnote 20}

This relationship between Photographs 49 and 51 emerges all the more clearly when Franklin’s notes about them are compared with earlier entries in her laboratory notebooks. She had by then established a note-taking routine well suited to the open-ended trials characteristic of Photographs 1–49, with two distinct parts for each entry. The first section, written before the photograph was taken, provided details of the experimental set-up, including the date and time when X-ray exposure commenced. Afterward, Franklin would record the date and time when she and Gosling ended the exposure and then complete a second section, for which she customarily left a few lines empty, giving a brief indication of how the experiment had turned out.

Initially, she wrote about Photograph 49 using her standard format. She first specified which DNA sample she was using, the relative humidity at which it was maintained, and the source of the X-rays. Three days later, she filled in the space below to report that this trial had produced a “V[ery] good ‘wet’ photo” (meaning a very good photo of the B form). However, she then turned to a fresh set of pages in the notebook and filled them with the analyses we discussed above. Headed with the very date the photograph had originally been developed, this passage of notes filled far more space in her laboratory notebooks than did the discussion of any single previous DNA diffraction pattern, indicating that she immediately found this version of the B pattern to be strikingly important.

The entry for Photograph 51 was unprecedented in a completely different way. This was the first instance in Franklin’s DNA laboratory notebooks where she was able to write down in advance what details the diffraction pattern would include (namely, both 3.4 Å arcs) and the first occasion when she specified that correcting the cropping of a previous photograph was the reason for taking a new one. As an understandable consequence, Franklin did not bother to fill the lines at the end of entry 51 where she would normally have described the result of the photograph. Indeed, the empty space stands out starkly on a pair of pages otherwise densely filled with the initial conditions and the results of the surrounding experiments. As discussed below, later commentators have misinterpreted this omission as the act of a person who had just conducted a potentially crucial experiment, but who didn’t appreciate its significance. Rather, as we have shown, Photograph 51 was not an experiment at all in the sense that trials 1 to 50 had been. It was, in fact, the product of Franklin’s decision to invest four days of X-ray time into a known outcome. The investment made sense precisely because she valued a high-quality photograph of the helical B form diffraction pattern.

Franklin and Gosling did not immediately publish Photograph 51. Later commentators, armed with the knowledge that it would be less than a year before Watson and Crick built their double helix as a physical model of the very B form structure Franklin and Gosling discovered and characterized, have criticized Franklin for directing much of their effort during the rest of 1952 to analyzing the A form. However, what critics describe as Franklin’s discovery — and subsequent apparent neglect — of evidence for the crucial B form structure of DNA looked different from her perspective and in the context of her broader research objectives. Franklin’s stated objectives, as we have seen, were to understand the relationship between and interconversion of the two forms of DNA. Photographs 49 and 51, and her analysis of the former, which revealed the key parameters of a helical structure, represented an important and successfully achieved milestone. It is only with hindsight that we view the B form structure as the only important goal, and the structure of DNA. DNA clearly had more than one structure and Franklin wanted to solve both of them.

By completing Photograph 51, Franklin now had good images in hand of both the A form and the B form. From here, she turned to the second major objective of her established research plan: detailed structural analysis beginning with the crystalline A form, whose pattern offered more data for analysis than the paracrystalline B form. The results of several months’ work led her to conclude that this crystalline structure was likely a double-chained molecule, and in February 1953 she turned back to Photographs 49 and 51 to assess whether the B structure in turn showed “evidence for [a] 2-chain […] helix.”^{Footnote 21} That the B form photographs indicated a helical structure however, was not in doubt, as indicated by her notebooks. By the end of February, as she was leaving King’s College for a position at Birkbeck College, she and Gosling had drafted three papers, two of which would contain Photograph 51 when they were published later that year.^{Footnote 22} Meanwhile, since Gosling was to remain at King’s to finish his PhD under Wilkins’ supervision, Franklin directed him to give Wilkins the diffraction photograph that Wilkins in turn showed Watson.^{Footnote 23}

For all the investment Franklin originally made to capture a publication-worthy version of the B pattern, when it came time to publish Photograph 51 she took yet another step to refine the image. In early March 1953, she turned over film 51C to be duplicated onto a durable glass slide negative that could, in turn, be used to produce positive photographic prints for submission.^{Footnote 24} The slide, which survives and is numbered 867 in the biophysics unit’s indexing system, became the basis for published reproductions.^{Footnote 25} This is evident because published images of Photograph 51 show the diffraction pattern centered within a circular border even more precisely than it had been when the pattern was originally developed on film. The perfected cropping was achieved by placing masking tape on the glass plate itself (see Fig. 5). The figures of Photograph 51 in both of Franklin and Gosling’s 1953 publications (1953a and 1953b) that included it show the ultra-refined cropping from the glass plate.