Whose photo revealed DNA's structure

You cannot photograph a DNA molecule: it is hundreds of times thinner than the wavelength of visible light. So it is exposed to X-rays. The beam passes through an ultrafine thread drawn from a viscous solution, scatters off the atoms, and leaves not an image of the molecule on the film but its imprint - a set of spots. Its shape is reconstructed from where those spots fall.

Photo 51 was taken in May 1952 by graduate student Raymond Gosling - using Rosalind Franklin's equipment, her method, and under her supervision. The key was not the exposure itself but the conditions. In dry air, the thread contracts into a dense A-form with a tangled pattern. Franklin learned to keep the chamber highly humid, and the thread stayed in the stretched-out B-form: there were fewer spots, but they could be read almost directly. One exposure lasted about 60 hours.

You have probably seen this image: a blurry diagonal cross made of spots. Scattering from any helix creates a cross on film - whether from a screw or a spring; the angle between the rays shows how tightly it is twisted. The distance between the horizontal rows of spots gives the length of one turn, while the dark spot at the top and bottom gives the gap between neighboring steps inside it. The missing fourth row means there are two strands, offset from each other.

In January 1953, Maurice Wilkins, who worked at the same college, showed the photo to James Watson; Franklin was not asked. In February, an internal lab report containing her measurements also reached Crick. Watson later wrote that he understood the key point almost as soon as he saw the cross - after that, they only needed the numbers.

On April 25, 1953, Nature published 3 papers in a row: Watson and Crick's model, work by Wilkins and colleagues, and Franklin and Gosling's paper with that very photo. From the outside, they looked like 3 independent results. The model's authors cited Franklin's unpublished data, but the citation did not show that the model was built on them. And she understood her own photo: a draft of her paper on the two strands is dated mid-March of that year. Franklin died in 1958, 4 years before the prize.