A CENTURY'S PROGRESS IN PHYSICS 357 



effect to be due to some new and unknown type of radia- 

 tion. Further investigation showed that this radiation 

 originates at the points where cathode rays impinge on 

 the glass walls of the tube. Besides being able to pass 

 with ease through all but the most dense material objects 

 X-rays were found to have the power of ionizing gases 

 through which they pass and ejecting electrons from metal 

 surfaces against which they strike. The points at which 

 these electrons are produced are in turn the sources of 

 secondary X-rays whose properties are characteristic of 

 the metal from which they come. 



Rontgen's discovery excited intense interest among 

 laymen as well as in scientific circles. Of the many 

 X-ray photographs taken, those of Wright (1, 235, 1896) 

 of Yale were the first to be produced in this country. 

 His experiments were made immediately on receipt of 

 the news of Rontgen's research, and resulted in the pub- 

 lication of a number of photographs showing the trans- 

 lucency for these rays of paper, wood, and even 

 aluminium. 



As X-rays are undeviated by electric or magnetic fields, 

 Schuster, and later Wiechert and Stokes, suggested that 

 they might be electromagnetic waves of the same nature 

 as light, but much shorter and less regular. The great 

 objection to this hypothesis was the failure either to 

 refract or diffract these rays. In fact Bragg contended 

 that they were not etherial disturbances at all, but con- 

 sisted of neutral particles moving with very high veloci- 

 ties. Finally Laue 11 demonstrated their undulatory 

 nature by showing that diffraction took place under 

 proper conditions. Just as the distance between adja- 

 cent lines of a grating must be comparable to the wave 

 length of light for a spectrum to be formed, a periodic 

 structure with a grating space of their very much shorter 

 wave length is necessary to diffract X-rays. Such a 

 structure is altogether too fine to be made by human 

 tools. Nature, however, has already prepared it for 

 man's use. The distance between the atoms of a crystal 

 is just right to make it an excellent X-ray grating, and 

 Laue had no difficulty in obtaining diffraction patterns 

 when Rontgen rays were passed through a block of zinc- 

 blende. The distance between adjacent atoms of this 



