ELECTRONIC MAGNITUDES 317 



What might be called the classical theories of radia- 

 tion have been upset by the remarkable accumulation 

 of evidence indicating the correctness of Planck's 

 hypothesis that energy is absorbed only in definite 

 quanta, each of value hf where / is the frequency. The 

 fact that it is so absorbed is evident from the photo- 

 electric effects, but the mode of its emission, transmis- 

 sion, and absorption is as yet in the domain of specula- 

 tive physics. For that reason we shall not attempt a 

 further discussion of radiation. Future scientific in- 

 vestigations and analyses will ultimately reconcile the 

 quantum theory with the portion of the classical 

 theory which is supported by direct experiment. One 

 of the difficult points is to reconcile the idea of inter- 

 ference of wave trains of continuous energy with the 

 present evidence of the granular nature of the energy. 

 The phenomenon of interference was responsible for 

 overturning that earlier corpuscular theory of light 

 which had the influential support of Newton. That 

 theory pictured light as a stream of corpuscles, while 

 the present theory may only definitely assert that the 

 energy which we recognize as light is to be received 

 by an electron oscillator only in definite quanta. 



With one application of the phenomenon of inter- 

 ference we are immediately interested because it is 

 involved in the determination of the electron numbers 

 which are referred to in Chapter VIII. Interference, 

 however, is a phenomenon common to all instances of 

 the transmission of energy from periodic sources. It 

 occurs in molecular media as well as in the ether. 

 Consider for example a row of similar sources as 

 indicated in Fig. 43. Along the line ad the magni- 



