Built alongside a companion piece on Ada Lovelace's Note G. The aim for both is to let the science and carefully checked history speak for itself.
This page sits next to an app on Ada Lovelace's Note G, part of a broader interest in women whose contributions to science and mathematics reward closer attention. Rosalind Franklin's place here doesn't need a dramatic claim to earn it. She took Photo 51, worked out that the DNA's phosphate backbone sits on the outside of the molecule rather than the inside, and had already measured a crystal symmetry that, correctly read, points straight at two strands running in opposite directions. The approach taken here is simply to build the real mathematics as a working app, check every historical claim as carefully as the equations, and let the pattern and the annotations speak for themselves.
Photo 51 is an X-ray diffraction pattern: X-rays fired at a DNA fibre scatter off the atoms inside it, reinforcing each other in some directions and cancelling out in others — the same wave interference behind an ordinary diffraction grating, just at a far smaller scale. Two features of DNA's shape combine to make the X. First, the helix's regular turn means the pattern splits into evenly spaced horizontal rows, called layer lines. Where the turn is tighter, the rows are further apart, just as a finer grating spreads light out more. Second, as the phosphate backbone winds around the helix, the angle it presents to the beam keeps changing, and this spread of angles pushes each row's brightest point further from the centre the higher or lower up that row sits. Evenly spaced rows, diagonally spreading brightness: together, that's the X, and nothing except a helix produces it.
The app's Viewpoint toggle sets the same photograph side by side against two different readings of it. Watson's reading is fast: a year spent studying a different helical virus had primed him to recognise the X almost on sight. Franklin's is slower and more rigorous: she had independently measured a symmetry in the crystal that implies two antiparallel strands, but hadn't yet drawn out that specific conclusion herself, and wasn't willing to publish before she had fuller evidence. Neither reading is presented as simply right or wrong. A third, Neutral view sits alongside both, trying to state only what the mathematics itself establishes, independently of either person's timing, training, or particular vantage point.
The documented sequence is straightforward: Wilkins showed Watson the photograph without Franklin's knowledge, and separately, Crick read the symmetry data in one of Franklin's own reports and worked out its implication himself. Watson and Crick's paper, Franklin and Gosling's, and a third by Wilkins's group appeared together in the same issue of Nature in April 1953 — though the way the issue was arranged left Franklin's paper reading as supporting evidence rather than an equal, independent discovery, despite her data sitting at the centre of it. It's worth asking, carefully, what might have happened had she drawn out that last connection herself. By her own account, by her own word, she came very close. There's no way to know for certain. But a reasonable version of that alternative history is a paper of her own, reaching much the same structure from her own data, at around the same time, or conceivably first.