To make an image of a molecule by x-ray diffraction, the pure substance must first be obtained in the form of a crystal. The crystal is then bombarded with x-rays, and the reflection—the angles at which the x-rays bounce back—is recorded on film. Rosalind Franklin made these two images of DNA, taken at different levels of humidity. The “X” shape of the lower image indicates a helical structure.

Credit: From The Double Helix by James D. Watson, 1968, Atheneum Publishers. James D. Watson Collection, Cold Spring Harbor Laboratory Archives.