252 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 19 30 



of a millimeter). In order to show the existence of the associated 

 electronic waves, it is necessary to try to realize with them phenomena 

 of interference analogous to those which we get with X rays. Phe- 

 nomena of this nature have been obtained. The first physicists who 

 succeeded in obtaining such results were Messrs. Davisson and Germer 

 of the Bell Laboratories in New York. But I am not going to de- 

 scribe their experiments; instead I will tell you of the beautiful 

 results, analogous in nature, although slightly different, which were 

 made later by G. P. Thomson in England and by M. Ponte in France, 

 for M. Ponte has had the kindness to lend me the photographs so 

 that I may lay before your eyes the actual results. 



When we send a pencil of X rays upon a powder composed of 

 small crystals oriented in random directions the X rays are diffused 

 by the regularly arranged molecules in the small crystals and inter- 



Powder of 

 Crystals 



Photographic 

 Film 



Figure 1 



ference takes place between the diffused X rays. Theory predicts 

 that if we place a photographic plate perpendicular to the pencil 

 of rays incident upon the plate in the manner shown in Figure 1, 

 the X rays should affect the photographic plate in circles centered 

 upon the direction of the incident ray. Experiment verifies splen- 

 didly the prediction and we have upon the plate rings which are 

 black upon the negative itself but white as shown in the positive 

 reproduced in Plate 1, Figure 1, 



The diameter of the rings depends upon the nature of the crystal- 

 line substance employed as a diffusing means and upon the wave 

 length of the radiation. If with the same diffuser we employ X-ray 

 radiations of shorter and shorter wave lengths the system of rings 

 moves toward the center. 



We may employ in the place of the powder of small crystals a very 

 thin metallic plate. The study of the structure of metals has shown 

 that they are formed of a conglomeration of small confused crystals. 



