146 



by Walter and Pohl 1 ), and theoretical speculations by Wien, 

 Stark, and others, had gradually led to the conviction that the 

 wave-length of R on t gen-radiation would be of the order of 10 8 

 or 10 9 cm. If this were true however, the wave-length would be 

 of the same order of magnitude as the interatomic distances supposed 

 in space-lattices of crystals, and in that case there would exist a 

 great probability that the crystalline medium would behave no 

 longer as a continuum towards Ron t gen-radiation. It might be 

 expected that the crystal would behave towards these extremely 

 short transversal waves in a way analogous to that, which the 

 wellknown "gratings" in optics do towards ordinary light-waves, 

 and that a diffraction-phenomenon, would occur, the nature of 

 which would be analogous to that which would be produced when 

 visible light fell upon a grating having three dimensions instead of 

 only two. 



21. The ingenious idea that such a crystal might be used as 

 a tridimensional and most perfect "grating" for Rontgen-rays, 

 was in 1912 conceived byVonLaue 2 ). The experiment was carried 

 out in the spring of 1912 by Friedrich and Knipping 3 ) in 

 Sommerfeld's laboratory, and it was crowned with complete 

 success. Round the central spot at the point where a thin pencil 

 of Rontgen-rays, after passing the crystal, met the photographic 

 plate, a great number of oval spots of different intensities were 

 produced, which were symmetrically arranged about it, when the 

 direction of the rays coincided with that of a symmetry-axis of the 

 crystal-plate. In the preceding chapters some of these R on t gen- 

 patterns have already been considered in detail, so that it is un- 

 necessary here to dwell upon them. 



Since then, an ever-increasing number of experimental and theo- 

 retical work has been published 4 ) concerning this highly important 

 phenomenon, which at the same time revealed the true nature of 



!) H. Haga and C. H. Wind, Wied. Ann. der Phys., (3), 68, 884, (1899); 

 (4), 10, 305, (1903); B. Walter and R. Pohl, Ann. der Phys., (4). 28, 715, 

 (1908); (4), 29, 331, (1909). 



2 ) M. Von Laue, Bayr. Akad. d. Wiss., (1912), p. 303; Ann. d. Phys., 

 42, 397, (1913). 



3 ) M. Von Laue, W. Friedrich, and P. Knipping, Ber. Bayr. Akad. 

 der Wiss., (1912), p. 303. 



4 ) H. Haga and F. M. Jaeger, Proceed. Kon. Akad. van Wet. Amster- 

 dam, Vol. 16, 17, 18, (19141916); P. Debije, Verh. der phys. Ges., 15, 678, 

 738, 857. (1913); P. P. Ewald, Sitz. kon. Bayr. Akad. d. Wiss., (1914), p. 325; 



