130 



SCIENCE 



[N. S. Vol. XXXVII. No. 943 



10"° centimeters, or .0001 that of the short- 

 est known ultra-violet waves. These ex- 

 periments present strong evidence that 

 some types of X-rays at least possess a 

 periodic character. In a word, then, all 

 these similarities suggest inevitably the 

 hypothesis that ordinary scattered X-rays 

 are white light, of short wave-length, and 

 that characteristic X-rays are monochro- 

 matic light of short wave-length. If 

 Bragg 's neutral pair theory is to have any 

 future, it must in all probability, then, be 

 extended to all electro-magnetic radiations. 



But how, when a charged pitch ball, for 

 example, swings back and forth on its silk- 

 thread suspension in our laboratories, are 

 the periodic electromagnetic disturbances 

 which it sets up in the neighborhood to be 

 interpreted in terms of the emission of 

 neutral pairs? No one is bold enough at 

 present to attempt to thus resurrect a 

 straight corpuscular theory of all ethereal 

 radiation, with all that it implies regard- 

 ing the dependence of the velocity of light, 

 on the velocity of the source, the interpre- 

 tation of interference phenomena in light 

 and of Hertz's wave phenomena in the 

 realm of wireless telegraphy. We need, 

 then, a more general hypothesis than that 

 of Bragg. 



4. Such a general hypothesis was made 

 by J. J. Thomson in his Silliman lectures 

 in 1903.'^ It was, historically, the first 

 form of the modern atomistic theories of 

 radiation as regards space relations, al- 

 though it is here treated in the fourth 

 place, because it stands fourth in the 

 violence of the assumptions involved. Like 

 Bragg 's theory, it postulates radiant 

 energy which is emitted by the source in 

 bundles or quanta, though no necessary 

 multiple relationship was at first assumed 

 between the different elements emitted by 

 the same source. It goes farther than 



^"Electricity and Matter," pp. 63 et seq. 



Bragg 's theory in endeavoring to reconcile 

 this quantum notion with the wave theory 

 by assuming a fibrous structure in the 

 ether, and picturing all electromagnetic 

 energy as traveling along Faraday lines of 

 force conceived as actual strings extending 

 through all space. This is nothing more 

 than a new picture of the structure of the 

 ether and one which is perhaps no more im- 

 possible than all its rivals. To the support 

 of such a hypothesis are brought all the 

 arguments urged for Bragg 's theory, while 

 the arguments which I have urged against 

 Bragg 's theory are removed. It may be 

 difficult, not to say repugnant, to some of 

 us to attempt to visualize the universe as 

 an infinite cobweb spun by a spider-like 

 creator out of threads that never become 

 tangled or broken, however swiftly elec- 

 trical charges may be flying about or how- 

 ever violently we enmeshed human flies 

 may buzz, but such is the hypothesis, and 

 the objections to it will be treated along 

 with those of the next and most concen- 

 trated form of quantum hypothesis. 



5. This was proposed by Einstein^* in 

 1905, and is simply the J. J. Thomson 

 theory of the discontinuous distribution of 

 radiant energy in space, assumed still to be 

 electromagnetic and hence to have a velocity 

 independent of that of the source, with the 

 addition of Planck's original assumption 

 that a given source emits and absorbs en- 

 ergy in units which are multiples of Jiv. 

 This amendment has apparently been ac- 

 cepted by Thomson and seconded by Lar- 

 mor. For the energy units, hv, have had 

 some experimental successes, the considera- 

 tion of which it was thought best to defer 

 to this point. 



In the normal photo-electrical effect the 

 kinetic energy of the escaping electron in- 

 creases with the frequency of the incident 



=•■ Ann. der Phys., 17, p. 132, 1905. 



