42 SCIENCE PROGRESS 



Now there are two methods according to which radiation 

 may spread away from a source and the two stand in apparent 

 antithesis to each other. One of them may be illustrated by the 

 spreading of the ripples on a water surface into which a stone 

 has fallen : the disturbance is distributed over ever-widening 

 circles and the height of the ripple diminishes accordingly. 

 If some small object is floating on the pond, the movement 

 which it experiences as the ripple passes under it is smaller 

 the farther away it is from the splash. This is not the kind 

 of spreading of radiation that we now require. We may 

 illustrate the other method by picturing to ourselves a number 

 of soldiers holding a small fort and shooting at enemies who 

 are on all sides of them. Here the bullets radiate from a 

 common centre and, of course, if we may suppose for the 

 purpose of our illustration that the shooting is quite at random, 

 the chance that a particular soldier of the enemy is hit is less 

 the further he is away from the fort. But if he is hit the force 

 of the bullet is practically the same as if he is close to. Hits 

 are less frequent at greater distances but they are as severe in 

 their results. It is this sort of radiation which the 7 rays 

 resemble. To distinguish it from the former we may term it 

 "corpuscular"; and the use of the word would imply that we 

 regard 7 rays as a stream of discrete entities, a flight of pro- 

 jectiles each one of which keeps its characteristics, such as form 

 and volume and energy, unchanged during its motion, until it 

 spends energy in the production of the /3 ray. We need not 

 at this stage define the corpuscle more closely than is necessary 

 to this idea : we are not bound so far to say whether it is 

 " material" or "immaterial " nor to try and find it amongst the 

 things with which we are already familiar. It is hardly 

 necessary to repeat that the X rays go with the 7 rays : the 

 two sets of rays differ only in degree and not in kind. 



We may take one more step forward which tends to sim- 

 plicity. As is well known, the X rays spring from a plate on 

 which cathode rays are falling. It is found that the cathode 

 rays which spring up under the action of the X rays have 

 the same speed as the original cathode rays in the X ray tube. 

 Perhaps we ought rather to say that we actually find some 

 of them possessed of the same speed and we infer that the 

 rest had it when they came into existence but have lost it, 

 as indeed we should anticipate, before we can bring them to 



