CONTEMPORARY ADVANCES IN PHYSICS 407 



we say that it was proved that X-rays possess some of the qualities 

 of waves. 



To make the spots appear a fluorescent coat or a photographic film 

 is spread on a flat screen, instead of the spherical bulb. The locations 

 on the screen can be deduced from those on the imaginary bulb by 

 simple projection. Different spots are likely to be due to components 

 of different wave-length in the primary beam, which will probably 

 not be equally intense — a thing to be remembered when deducing the 

 diffraction-pattern of the atom-group. 



Figs. 8, 9 — Diffraction-spots ("Laue patterns") obtained when a beam in- 

 cluding waves of many wave-lengths is directed against a fixed single crystal. These 

 are the historic patterns obtained with X-rays and a zincblende crystal by Friedrich 

 and Knipping. The two correspond to different orientations of the crystal relative 

 to the primary beam. 



(IV) If the incident beam is monochromatic or nearly so we can 

 present to it not a single stationary crystal but a confused mass of 

 tiny crystals oriented in every way whatever. The desired coincidence 

 will certainly occur for some among the crystals. 



This is the "powder method" invented and applied to X-rays by 

 Hull and by Debye and Scherrer, and applied to electron-waves by 

 G. P. Thomson. The term implies that the chaotic mass of little 

 crystals is obtained by pulverizing a large one; but small pieces or 

 thin films of ordinary metals are likely to present quite as complete a 

 chaos. The diffraction-pattern when formed on the wall of the bulb 

 or on a flat screen set normal to the direction of the primary waves, 

 consists not of separate spots but of continuous rings (Figs. 17-20). 



Such in outline are the four great methods for the analysis of waves 

 by crystals and of crystals by waves. Each has its own field; each 



