Although the periodic structure of solids had been proposed by crystallographers in the 19 th century, atomic scale experimental validation was lacking. In 1912, M. V. Laue, W. Friedrich, and P. Knipping published the first diffraction pattern of a crystalline solid, zinc sulfide . This experiment and the latter interpretations from Bragg, Lau, Sommerfeld, and Ewald, among others, would settle the nature of solids and X-rays: most solids consist of a periodic arrangement of atoms, and X-rays are electromagnetic radiation of sufficiently short wavelength to diffract within such periodic structure. In 1913, W. L. Bragg described the diffraction problem as the reflection of X-rays from planes of atoms . X-rays constructively interfere when the path traversed by adjacent X-rays reflected from the atomic planes is equal to a multiple of their wavelength.