62 



NATURE 



[September 8, 192 1 



(c) does not depend on the nature of the metal ; 

 and 



(d) is equal to the velocity of the cathode rays in 

 the X-ray bulb. 



The last fact is a very striking one. To quote 

 from the Robert Boyle lecture recently delivered 

 by Sir William Bragg at Oxford (Nature, May 

 19, P- 374) •• 



It is not known how the energy of the electron 

 in the X-ray bulb is transferred by a wave motion to 

 an electron in the photographic plate, or in any other 

 substance on which the X-ray falls. It is as if one 

 dropped a plank into the sea from a height of loo ft., 

 and found that the spreading ripple was able, after 

 travelling looo miles and becoming infinitesimal in 

 comparison with its original amount, to act upon a 

 wooden ship in such a way that a plank of that ship 

 flew out of its place to a height of loo ft. How does 

 the energy get from the one place to the other? 



According to the discussion of these difficulties 

 usually given, the X-rays should spread equally 

 in all directions according to the wave theory. 

 Consequently the few molecules which, according 

 to (i), become ionised should be in some excep- 

 tional condition, i.e. have a high kinetic energy. 

 But this energy cannot be ordinary heat energy, 

 since according to Xp) ionisation does not vary with 

 temperature. It must be some store of internal 

 .energy of a radio-active nature, not readily un- 

 locked by outside agencies. The X-ray thus acts 

 as a trigger to start an explosion, of which the 

 corpuscle is the outward and visible sign. 



Why should the speed of the corpuscles depend 

 on the wave-length of the X-ray, if the latter 

 merely exerts a trigger action, and why is it inde- 

 pendent of the distance of the X-ray bulb? Why 

 is it independent of the nature of the metal? The 

 difficulty of answering these questions in a satis- 

 factory manner has led many to believe that the 

 energy of the X-ray does not spread equally in all 

 directions, but travels in a straight line, and is 

 handed over completely to one corpuscle. Thus we 

 had Bragg's corpuscular or neutral-pair theory of 

 the X-ray and Einstein's quantum theory. 



The case for a localised energy theory or entity 

 theory, as it is called, for there are different 

 names, appears at first sight to be strengthened 

 by the numerical values of the quantities involved. 

 The velocity of the cathode-ray particle, the X-ray 

 it produces, and the velocity of the secondary cor- 

 puscle are connected by the equation 



where ra is the mass of the cathode-ray particle or 

 secondary corpuscle, v its velocity, h Planck's con- 

 stant, and V the frequency of the X-ray. Not only 

 is there agreement between the value of h deter- 

 mined from this equation and the value derived 

 from experiments on black body radiation, but 

 there is extremely g-ood agreement. In an article 

 in the Journal de Physique for August, 1920, by 

 M. de Broglie, who has himself added very con- 

 siderably to our knowledge of both emission and 

 absorption X-ray spectra, there is a description of 

 the methods and a collection of results, from 



NO. 2706, VOL. 108] 



which it is evident that the differences are less 

 than I per cent. 



References have been made to Newton's 

 emission theory. It is difficult for students to 

 study this theory, as the " Opticks " has not been 

 printed in English since 1730. A reprint is long 

 overdue ; the book is really accessible only in the 

 German, in Ostwald's " Klassiker." There are 

 quite erroneous ideas prevalent about the utility 

 of Newton's emission theory for present-day pur- 

 poses. Newton himself was not enthusiastic about 

 it, and was cautious about using it. In the first 

 sentence of the " Opticks " Newton says : " My 

 design in this book is not to explain the Proper- 

 ties of Light by Hypotheses, but to propose them 

 and prove them by reason and experiment," and he 

 is very careful in pursuance of this design to make 

 his statements as free from hypotheses as possible. 

 Again and again he uses the word " ray " when it 

 is obvious the thought in his mind is " stream of 

 particles constituting the ray." But he eventually 

 committed himself quite definitely. Light consisted 

 of streams of particles, the violet rays being the 

 smallest particles and the red rays the largest : the 

 glass of the prism attracted the particles in the 

 ray incident on it, and this attraction caused the 

 deviation of the ray : the smaller particles were 

 attracted more strongly than the larger particles, 

 and consequently suffered a greater deviation : 

 hence the formation of the spectrum. In order to 

 explain interference Newton found it necessary to 

 assume that the particles underwent periodic 

 changes of state. These changes he referred to as 

 " fits of easy reflection," because at the changes 

 the particles were reflected more easily ; the fits 

 occurred at equal intervals along the path of the 

 ray, and the length of these intervals varied with 

 the colour; for the " rays which paint the colour 

 in the confine of yellow and orange " the interval 

 of the fits was 1/89,000 part of an inch, i.e. 

 2'8 X io~^ cm. 



But, though Newton adopted the emission 

 theory, he had misgivings on the subject. This is 

 especially evident from a query, No. 13, which he 

 appends to the end of his " Opticks " as a problem 

 suitable for future investigation. This query 

 begins, "Do not several sorts- of rays make vibra- 

 tions of several bignesses, which according to their 

 bignesses excite sensations of several Colours, 

 much after the manner that the vibrations of the 

 Air, according to their several bignesses, excite sen- 

 sations of several sounds? " Then he suggests 

 that the violet rays make the shorter vibrations, 

 and the red ones the longfer vibrations. 



Newton's theory is, of course, so hopelessly in- : 

 adequate for the explanation of interference, dif- | 

 fraction, changes of phase and amplitude, resolv- I 

 ing power, etc., that no one has ever seriously , 

 thought of applying it to these fields of work. ! 

 It even gives a wrong: value for the pressure of ' 

 light, a subject in which one would expect it to be j 

 at an advantage. The wave theory, on the other . 

 hand, holds an extremely strong position in these ; 

 fields, a position much stronger than is imagined, ; 

 for few workers at present experiment on such . 



