55^^ 



NATURE 



[Januarit i6, igi^ 



X-radiation in one important particular. Investiga- 

 tors have gone furtlier. They have siiown with con- 

 siderable probability that the velocity of the ejected 

 electron varies with the wave-length of the light ; 

 the shorter the wave-length, the swifter the electron. 

 Moreover, there are lines of reasoning, worked out in 

 great detail by Planck, Einstein, and others, which 

 lead to the thought that light energy is contained in 

 separate quanta; the shorter the wave-length, the 

 more energy in the quantum. This is one of the most 

 remarkable developments of modern physics. 



It seems as if there was a strong invitation to con- 

 sider radiation from this point of view. We ought 

 not to think that in doing so we abandon the wave 

 theory or its electro-magnetic development. Rather 

 we might say that the radiation problem is too great 

 to be seen all at once from any point to which we 

 have hitherto attained, and that it is to our advantage 

 to look at it from every side. 



It would be quite fair, moreover, to say that there 

 is something after all in the corpuscular theory of 

 light. There is a very great deal of evidence, as I 

 have already indicated briefly, for a corpuscular theory 

 of X-rays ; and it is widely held that the two forms 

 of radiation are' akin to each other. How can we 

 hold a corpuscular and a wave theory of light at the 

 same time? 



If we say that radiation consists in the emission of 

 quanta, each of which traverses space without spread- 

 ing or altering in any way, and label this a cor- 

 puscular theory; and if, on the other hand, we sup- 

 pose light to consist of wave-motions, and that we 

 can resolve such wave-motions at any one time into 

 elements each of which might e.xist alone and would 

 then spread through space like a ripple on a pond ; 

 and if we say that the quantum in the one theory is 

 to be matched with the element in the other, then, of 

 course, the two theories are inconsistent. 



But such inconsistencies are difficulties of our own 

 making. If one hypothesis links together a number 

 of observed facts, and a second hypothesis a somewhat 

 different number ; and if we think the two are incon- 

 sistent, the fault must be ours. We must be stretching 

 one or other hypothesis to breaking-point, and we 

 must work in the hope of finding a new hypothesis of 

 greater compass. Until we do so, we are right to 

 use those which are more limited ; it is the way of 

 scientific advance. So the great men of the past have 

 done, as we may see readily. 



Let us go back to the discussions of the close of 

 the seventeenth century, the time when Newton, 

 Huygens, Hooke, Pardie, and others debated the 

 nature and form of light. .A very important discovery 

 had recently been made by Romer, who had shown by 

 astronomical observations that light, which brought 

 the news of the events taking place in space, took 

 time to bring it ; in others words, that light had a 

 velocity. Romer had even succeeded in measuring 

 the velocity with fair accuracy. Now Descartes had 

 supposed the propagation of light to be instantaneous. 

 He had considered it to be a pressure transmitted 

 across a plenum between the luminous object and the 

 eye ; according to a well-known image, vision re- 

 sembled the process by which objects are made mani- 

 fest to a blind man, who feels for them with his stick 

 and receives pressures transmitted thereby. Apart 

 from the direct proof by Romer that this view was 

 wrong, a very interesting objection to it is given by 

 Huygens, who, after stating the Cartesian theory, 

 remarks that "it is impossible so to understand what 

 I have been saying about two persons mutuallv seeing 

 one another's eyes, or how two torches can illuminate 

 each other." That is to say, it is impossible to explain 

 on a simple pressure theory the perfect facilitv with 



NO. 2255, VOL. 90] 



which rays of light traverse each other without injury. 

 This mutual traverse of light rays inieresltd Huygens 

 exceedingly, and, as we shall see, influenced materially 

 his choice of the hypothesis in terms of which he 

 expressed the facts known to l"um. 



Thus Newton and Huygens were led to introduce 

 the idea of motion of some sort of matter as a funda- 

 mental point in their theories. They did so in different 

 ways ; and the distinction grew to be a cleavage 

 between two schools of thought. It was not a very 

 deep distinction at first ; it would have been easy to 

 have stepped from one side to the other of the dividing 

 line. Only in later times did the corpuscular and 

 wave theories stand immovable in hostile antagonism. 

 It is not at all impossible that modern research will 

 once more draw the two theories together. 



The difference may be put in this way : — Newton 

 imagined light corpuscles which moved in straight 

 lines from the source of light to the recipient, 

 thought ihat the "light" had the same carrier from 

 beginning to end of its path. We should now express 

 his idju by saying that the "energy" of the light 

 had the same carrier ; but Newton did not, of course, 

 conceive of light-energy as a quantity to be measured 

 and discussed. How far he was from this more 

 modern idea is instanced by his supposition that the 

 radiating power of the sun was conserved by the 

 mutual radiation of its parts. 



On the other hand, Huygens imagined the light to 

 be passed on from particle to particle of the aether; 

 that is to say, the energy was not carried by one 

 particle all the way, but by relays. It must be remem- 

 bered that he thought of the cether as a collection of 

 particles resembling the particles of luminous bodies, 

 but of smaller dimensions. The latter particles he 

 supposed to float in a subtle medium which agitated 

 them and made them strike against the particles of 

 the aether, which thus became the seat of spreading 

 impulses. We might compare the difference between 

 the ideas of Newton and Huygens with the difference 

 between the despatch of a message by a special runner 

 and the spreading of a rumour. 



Huygens has given two reasons for his choice of 

 hypothesis : one, the extreme speed of light, which 

 Romer had recently found ; the other, the ease with 

 which rays of light traverse each other. These in- 

 duced him to reject the idea of the movement of 

 matter through the whole of the distance from source 

 to receiver, since he could not imagine how matter 

 could move with so great a speed, nor could he con- 

 ceive how material rays could pass through each 

 other. He arranged «ther corpuscles in a row between 

 source and receiver, and supposed light to move along 

 the row in the same way that a disturbance would 

 move along a row of glass spheres placed so as to 

 touch each other. Indeed, he filled all space with <Ether 

 particles in contact so as to allow of tHe transmission 

 of disturbances from any one point to any other. In 

 such ^ a plenum two disturbances might easily be 

 imagined to cross each other without hindrance. To 

 use an illustration which he gives himself, " If against 

 this row (B C in Fig. 10) there are pushed from two 



CXXMtCO 



opposite sides at the same time two similar spheres 

 .'\ and D, one will see each of them rebound with the 

 same velocity which it had in striking; vet the whole 



will remain in its place, although the movement 

 passed along its whole length twice over." It is. 



row wi 



has passed along ..., ,...v,.^ .^..f^.,. iw.^c ^^.>... ^^ .^, 

 of course, the movements, not the spheres, which 



