200 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



work are mentioned, the existing means of experimentation are out- 

 lined, and some of its present limitations are discussed, together with 

 some of the kinds of problems to which a knowledge of the arrange- 

 ment of the atoms in crystals has contributed and may be expected 

 to contribute. 



AN OUTLINE OF THE DEVELOPMENT OF THE MEANS EMPLOYED IN STUDYING 

 THE STRUCTURES OF CRYSTALS. 



The experiment of Laue. — For some time it has been evident that 

 the distance apart of the atoms in a solid body is of the order of 10' 8 

 centimeters. 2 In the days before the nature of X rays was known, 

 attempts were made to see if they could be diffracted by passage 

 through very narrow slits; the results of these experiments showed 



that if X rays re- 

 ally were wave mo- 

 tions of a nature 

 similar to ordi- 

 nary light, their 

 wave lengths could 

 not by much ex- 

 ceed 10 -9 centi- 

 meters. 3 



Starting from 

 this information 

 Laue concluded 

 that if X rays are 

 wave motions of 

 this type they 

 should be dif- 

 fracted on passing through an orderly arrangement of atoms such as 

 is furnished by a crystal ; and, as a matter of fact, when a narrow pen- 

 cil of X rays was passed through a thin section of a crystal, a number 

 of diffracted images of the pinhole defining the beam were obtained, 

 arranged in a symmetrical fashion about an undiffracted image. 4 The 

 arrangement required for carrying out this experiment is shown in 

 figure 1. A beam of X rays after passing through two pinholes in the 

 lead screens A and B proceeds through the thin section of a crystal at 

 C and registers itself as the undeviated and diffracted images upon a 

 photographic plate placed at D. The kind of diffraction patterns 



X-rays 



Fig. 1. 



8 An indication of the dimensions of the " spheres of influence " of atoms has been ob- 

 tained in many ways, as from the thickness of soap-bubble films, from calculations based 

 on the kinetic theory of gases, and more especially from the work of Perrin on the 

 Brownian movement and from counts of a particles. 



a B. Walter u. R. Pohl, Ann. d. Pliys., 25, 715, 1908; 29, 331, 1908. R. Sommerfeld, 

 ibid., 38, 473, 1912. P. P. Koch, ibid, 38, 507, 1912. 



* M. Laue, W. Friedrich, u. P. Knipping, Ann. d. Phys., 41, 971, 1913. 



