226 



DISCOVERY 



sliip. It may liavi' I'licountirctl tliousaiuls of ships 

 before that and notliing lias happened, Init in this one 

 particular case the unexpected happens. One of the 

 ships timbers suddenly flies up in the air to exactly 

 100 feet, that is to say, if it got clear away from the 

 ship without having to crash through parts of the 

 rigging or something else of the structure. The problem 

 is, where did the energy come from that shot this plank 

 into the air, and why was its velocity so exactly' 

 related to that of the plank which was dropped into 

 the water i,ooo miles away ? It is this problem that 

 leaves us guessing. 



Shall we suppose that there was an explosive charge 

 in the ship ready to go off, and that the ripple pulled the 

 trigger. If we take this line of explanation we have to 

 arrange in some way that there are explosive charges of 

 all varieties of strength, each one ready to go off when 

 the right ripple comes along. The right ripple, it is to 

 be remembered, is the one whose frequency multiplied 

 by the constant factor is equal to the energy set free 

 by the explosion. The ship carries about all these 

 charges at all times, or at least there are a large number 

 of ships each of which carries some of the charges, and 

 externally the ships are exactly alike. Also we have to 

 explain why, if we may drop our analogy and come back 

 to the real thing, the ejected electron tends to start its 

 career in the direction from which the wave came. 

 This is a very marked effect when the waves are very 

 short. 



Dropping the analogy, how do the electrons acquire 

 their energy and their direction of movement from waves 

 whose energy and momentum have become infinitesi- 

 mally small at the spot where they are affected, unless 

 the atom has a mechanism of the most complicated 

 kind ? And if the intervention of the atom is so 

 important, why is it that in these effects a consequence 

 of the intervention does not depend upon each atom 

 itself — whether, for example, it is oxygen or copper or 

 lead ? 



We may try another line of explanation and suppose 

 that the energy is actually transferred by the wave 

 from the one electron to the other. If it is the atom 

 which pulls the trigger and causes the transforma- 

 tion, then how does it happen that the whole of the 

 energy collected by the wave at its origin can be 

 delivered at one spot ? Rayleigh has told us that an 

 electron over which a wave is passing can collect the 

 energy from an area round about it whose linear dimen- 

 sions are of the order of the wave length. But any 

 explanation of this kind is entirely inadequate. What- 

 ever process goes on, it is powerful enough on occasion 

 to transfer the whole of the energy of the one electron to 

 the other. Nor can there be any question of storing 

 up energy for a long period of time until sufficient is 

 acquired for the explosion. For it is not difficult to 



show that when an X-rav bulb is started and its rays 

 radiate out, the actual amount of energy which can be 

 picked up by an atom a few feet away would not be 

 sufficient for the ejected electron, though the tube were 

 running for months ; whereas we find the result to be 

 instantaneous. 



I think it is fair to say that in all optical questions 

 concerned with the general distribution of energy from 

 a radiating source the wave theory is clearly a full 

 explanation. It is only when we come to consider the 

 movements of the electrons which both cause waves 

 and are caused by them that we find ourselves at a loss 

 for an explanation. The effects are as if the energy 

 were conveyed from place to place in entities, such as 

 Newton's old corpuscular theory of light provides. 

 This is the problem for which no satisfactory solution 

 has been provided as yet ; that, at least, is how it 

 seems to me. 



No known theory can be distorted so as to provide 

 even an approximate explanation. There must be some 

 fact of which we are entirely ignorant and whose dis- 

 covery may revolutionise our views of the relations 

 between waves and ether and matter. For the present 

 we have to work on both theories. On Mondays, 

 Wednesda}s, and Fridays we use the wave theory ; 

 on Tuesdays, Thursdays, and Saturdays we think in 

 streams of flying energy quanta or corpuscles. That 

 is, after all, a very proper attitude to take. We cannot 

 state the whole truth since we have only partial 

 statements, each covering a portion of the field. When 

 we want to work in any one portion of the field or 

 other, we must take out the right map. Some day we 

 shall piece all the maps together. 



Meanwhile, even if we cannot explain the phenomena, 

 we must accept their existence and take account of them 

 in our investigations. We must recognise that wave 

 radiation and electron radiation are in a sense mutuaUy 

 convertible. Whenever there is one there must be the 

 other, provided only there is matter to do the trems- 

 forming. \Ye do not yet know more than a little of the 

 part that this process of interchange plays, but we know 

 that it is very prominent when the waves are very short, 

 or, what is the same thing, the electrons mo\-ing swiftly. 

 It is the movement of the electrons in the X-ray bulb 

 that originates the X-ra\-s themselves. The\- as waves 

 pass easily through the wall in the tube and through 

 materials outside ; their energy finally disappears and 

 is replaced bj' moving electrons. It is the latter alone 

 that produce directly the effects which we tiscribe to 

 X-raj-s. We may suspect that similar effects to these 

 take place when the waves are long, but the correspond- 

 ing electron velocities are so small that it is difficult 

 to measure them or observe their effects. Nevertheless, 

 the carrying forward to these regions of experience 

 gained elsewhere has led to extraordinary results, as. 



