184 ANNUAL REPORT SMITHSONIAN INSTITUTION, 195 



was also familiar with jets of particles, called corpuscular rays, such 

 as cathode rays, beta-rays, alpha-rays, anode rays, etc. Particles 

 would emit and absorb waves. For instance, cathode rays (electrons) , 

 when slowed down by colliding with atoms, emit X-rays. The dis- 

 tinction between particles and waves was, however, considered as 

 clear-cut as that between a violin and its sound. An examinee who 

 alleged cathode rays to be waves, or X-rays to be jets of particles, 

 would have got very bad marks. 



In the new setting of ideas the distinction has vanished, because it 

 was discovered that all particles have also wave properties, and vice 

 versa. Neither of the two concepts must be discarded ; they must be 

 amalgamated. Which aspect obtrudes itself depends not on the 

 physical object, but on the experimental device set up to examine it. 

 A jet of cathode rays, for example, produces in a Wilson cloud chamber 

 discrete tracks of water droplets — curved tracks if there is a magnetic 

 jBeld to deflect the electrons, otherwise straight alignments of droplets. 

 We cannot but interpret them as traces of the paths of single elec- 

 trons. Yet the same jet, after crossing a narrow tube placed at right 

 angles to it and containing crystal powder, will produce on a photo- 

 graphic plate at some distance behind the tube a pattern of concentric 

 circles. This pattern can be understood in all its details when looked 

 upon as the interference pattern of waves, and in no other way. In- 

 deed, it bears a close resemblance to similarly produced X-ray 

 patterns. 



The suspicion arises: are the conical jets that impinge on the 

 photographic plate and form the pattern of circles really cathode 

 rays; are they not perhaps secondary X-rays? The suspicion has 

 to be dismissed, for the whole system of circles can be displaced by 

 a magnet, while X-rays cannot; moreover, by putting a lead screen 

 with a small hole in it in the place of the photographic plate, a jetlet 

 can be isolated from one of the conical jets and made to display any 

 of the typical particle characters of cathode rays; it will produce 

 discrete tracks in a cloud chamber; bring about discrete discharges 

 in a Geiger-Miiller counter ; and charge up a Faraday cage in which 

 it is intercepted. 



A vast amount of experimental evidence clinches the conviction that 

 wave characteristics and particle characteristics are never encountered 

 singly, but always in a union ; they form different aspects of the same 

 phenomenon, and indeed of all physical phenomena. The union is not 

 a loose or superficial one. It would be quite unsatisfactory to consider 

 cathode rays to consist both of particles and of waves. In the early 

 days of the new theory it was suggested that the particles might be 

 singular spots within the waves, actually singularities in the meaning 

 of the mathematician. The white crests on a moderately rough sea 



