44 SCIENCE PROGRESS 



to the facts that among X ray phenomena there are one or 

 two which are more readily expressed in terms of wave motion 

 than of projectiles : whilst among light phenomena there are 

 some which strongly resemble the effects of " corpuscular " 

 motions. The X rays may be polarised in so far that a pencil 

 may be more liable to deflection in one direction than in 

 another ; and the effect is qualitatively what a wave theory 

 would lead us to expect. But the resemblance to polarised 

 light stops short at this. Moreover, primary X rays can excite 

 a secondary X radiation in certain substances which is less 

 penetrating than the primary and is characteristic of the 

 substance rather than of the exciting rays, reminding us of 

 fluorescence in the case of light. On the other hand light 

 can cause electrons to be ejected from substances on which 

 it falls ; they move very slowly and can be classified with the 

 S rays. According to recent experiment they seem to show 

 the effect we have met with above ; they continue to some 

 small extent in the direction of motion of the radiation to 

 which they are due. 



These parallelisms are very interesting and striking; and 

 when we consider them along with the simple and satisfactory 

 way in which the great bulk of X and 7 ray phenomena are 

 explained by a corpuscular theory and the difficulties into 

 which we are plunged by a corpuscular theory of light (it has 

 been reckoned one of the great achievements of the nineteenth 

 century to have escaped from them) we can only say that this 

 section of modern physics is in a very interesting position. 

 Are we to say that Newton and Huyghens were both right? 

 Shall we some day be able to understand that corpuscular 

 and wave radiations are only different aspects of the same 

 thing ? Or are we to adopt a different view ; to say that 

 a, /3, 7 and X radiations are all corpuscular, this common 

 property explaining their common characteristics, that light 

 as a wave motion stands apart and that the resemblances 

 described above are but accidental ? It works very well to 

 suppose that the 7 and X rays are only electrons with enough 

 positive electricity added to them to neutralise their negative 

 charge and so allow them to slip easily through electric fields 

 inside the atoms which would have stopped them otherwise; 

 the positive addition must be supposed to bring very little 

 mass with it. On this view, the progress of the X ray is 



