392 BELL SYSTEM TECHNICAL JOURNAL 



though the knowledge was at that date very recent, that the number 

 of atoms in a cubic centimeter of an ordinary crystal is such that 

 the average distance between them must be of that same order. 

 It was also known, as it had been for many years, that the atoms of 

 a crystal must be arranged in a regular order — a pattern, a network, 

 or a lattice; like soldiers on parade, except that the atoms parade in 

 three dimensions instead of two. Laue was aware of all these facts; 

 and one day it occurred to him, taking them all together, that if a 

 beam of X-rays was truly wave-like a crystal would diffract it — 

 would split it into a multitude of diverging beams, themselves grouped 

 in a pattern so curious and so symmetrical that if such a pattern 

 were indeed observed it could not be fortuitous, but by itself must 

 prove the assumption. 



The experiment was performed by two of Laue's colleagues on 

 "the experimental side," Friedrich and Knipping, with a crystal of 

 zincblende and a beam of X-rays from an ordinary X-ray tube. 

 The multitude of diverging beams made their appearance: the diffrac- 

 tion pattern was exactly as predicted. 



From this magnificent point of departure the advance was early 

 and rapid, in two directions. Crystals being able to diffract X-rays, 

 the phenomena could be used as sources of information either about 

 the rays or about the crystals. The former field was dominated the 

 sooner. In the fifteen years since the discovery, the technique of 

 using crystals to analyze X-ray spectra and measure the wave-lengths 

 of X-rays has been carried near to perfection. Nearly all the rays 

 which atoms can emit from their electron-shells, many of those which 

 proceed from their nuclei, have now been measured ; and the advantage 

 to atomic theory is immense. It is true that one can no longer say 

 that except for diffraction by crystals we should not know that the 

 X-rays are wave-like or what their wave-lengths are ; for now physicists 

 are beginning to map the X-ray spectra with optical ruled gratings. 

 But the crystals were the first to present us with these data, and in 

 most cases, I suppose, they are still the best. 



By contrast, the field in the other direction — the exploration of the 

 arrangement of atoms in crystals by means of their diffraction patterns 

 — seems unlimited. Newton's "ocean of undiscovered truth" is 

 not too strong a metaphor. The crystalline state, it transpires, is 

 universal. It is not confined to the lovely glassy specimens of the 

 mineralogical museum, with their smooth facets, sharp edges and 

 pointed pyramids — the jewels of Nature, from which the jewels of art 

 are made by perfecting or perverting the original design. The vivid 

 geometry of such as these is a signal of a regular, a "crystalline" ar- 



