The B-DNA preset, at its default settings, reconstructs the pattern in Franklin's photograph. Franklin had already recognised the helix, months before Watson's glance at this image — the real contrast is in what each did next: she turned to firmer evidence, he went straight to model-building. Use the viewpoint toggle above the pattern to see why.
DNA
TMV
DNA
TMV
DNA/TMV
both 1×
X-ray diffraction pattern on film — Photo 51 pattern
more layer linesfiner radial detail
The helix itself — real space, not film (projected, 1.5 turns shown)
This is not a measurement — it's the molecule these same sliders imply, drawn directly from r, P and Δφ. Horizontal and vertical scales are each auto-fit for legibility, so they are not comparable to each other or across presets. The small pale dots are bases, spaced ten per turn from the current pitch; with two strands, the faint rungs connecting them show which base on each strand pairs with which. Hover any point on the pattern above: the larger white dots here mark the real-space points that dominate that exact spot (the stationary-phase points, where cosφ = n/x) — strictly, every point on the helix contributes something, but these are the ones doing most of the work.
The maths, with your numbers in it
Hover the pattern to inspect a point.
Each row is one layer line, from Jn(2πrρ) at the effective radius, positioned at height ∝ n / effective pitch. Row shape is normalised for visibility; with 2 strands, each row's overall brightness is additionally scaled by cos²(nΔφ/2) — watch a whole row fade as Δφ sweeps through its extinguishing angle.