PROBLEMS OF MODERN SCIENCE 



and photoelectric effects. It is difficult to formu- 

 late the quantum theory accurately within the 

 compass of a nutshell, and in fact it is still not 

 quite certain just what its kernel consists of. This 

 much at any rate seems certain, that in the inter- 

 actions between radiation and matter which result 

 in the transference of energy from the radiant to 

 the material form or vice versa there are processes 

 which take place discontinuously and are incapable 

 of description in mechanical terms. The basis 

 of the quantum theory, however, appears to be 

 broader than this. It has been variously formu- 

 lated by different authors. I prefer to regard it as 

 a restriction on the systems which are dynamically 

 possible. Such a formulation has been given by 

 Dr W. Wilson of this College, and has been very 

 successful in the recent developments of the sub- 

 ject, particularly in the hands of Continental 

 writers. According to him, the dynamically pos- 

 sible systems are restricted to those for which 

 twice the value of the integral of the kinetic energy 

 with respect to the time taken over a complete 

 period is equal to an integral number n times a 

 universal constant, /i y known as Planck's constant 

 Ordinary dynamics lays no such restriction on the 

 motions which are possible. 



One of the great triumphs of the quantum 

 theory has been its use in the foundation by Bohr 

 of a theory of the emission spectra of the elements. 

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