THE STORY OF COSMIC RAYS — SWANN 



247 



to the energy after the occurrence in question gives a sum less than that 

 obtained by adding up the various contributions before the occurrence. 

 And so the concept of the neutrino was invented to play the role of 

 the thief who stole the energy. 



Photons. — The term "photon" is used to characterize all of those 

 particles that are associated with wavelike properties. In particular, 

 it comprises ordinary light rays, ultraviolet rays, X-rays, and the so- 

 called gamma rays from radium. 



There was a time when these radiations were thought to be of a wave 

 nature, the wave concerned spreading out into space from the place 

 of origin with ever-decreasing intensity. 



Figure 1, — These sketches illustrate the two concepts of the nature of light. At the right, 

 waves diminish in intensity as they spread from a source; at the left, wavelike particles 

 retain their energy as they travel outward. 



Early in the present century, radiations of this type began to present 

 a great puzzle to the student of physics. In some respects the radia- 

 tions acted like waves, as already stated, but in others they acted like 

 particles. 



The concept of spreading waves is inevitably bound up with dim- 

 inution of the wave's intensity with distance from the point of its 

 origin. On the other hand, the radiations we have classed as photons, 

 while exhibiting wavelike characteristics in certain phenomena, mas- 

 querade in other instances like bullets shot from a gun. Apart from 

 effects resulting from the resistance of the medium, such a bullet is 

 just as potent after traveling 100 yards as it is after 1 yard. In the 

 case of a battery of such guns firing in all directions from a fort, the 

 chance of getting hit would diminish with the distance from the fort. 

 But if one did get hit, he would be just as dead if hit at half a mile as 

 he would be if hit at 100 feet. 



The mathematical physicist has formulated his ideas and theories 

 in such a manner that he is not disturbed by the apparent paradox in 



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