L. Page — A Century's Progress in Physics. 349 



inexact to any such extent as the divergence between 

 theory and experiment would indicate, is inconceivable ; 

 that they are insufficient when applied to motions of elec- 

 trons in such intense fields as occur within the atom 

 seems no longer open to doubt. In order to obtain a 

 radiation formula in accord with experiment Planck has 

 found it necessary to extend the atomic idea to energy, 

 which he conceives to exist in multiples of a fundamental 

 quantum hv, v being the frequency and h Planck's con- 

 stant. That some such hypothesis of discontinuity is 

 essential in order to obtain any law that will even 

 approximately fit the experimental facts has been proved 

 by Poincare. But the precise spot at which the quantum 

 is introduced differs for every new derivation of Planck's 

 law. As deduced most recently by Planck himself, the 

 quantum shows itself in connection with the emission of 

 energy by the material oscillators with which the radi- 

 ation field is in equilibrium. These oscillators are sup- 

 posed to act quite normally in every respect except 

 emission; here the radiation demanded by the electro- 

 dynamic equations is cast aside, and an oscillator is 

 supposed to emit at once all its energy after it has accu- 

 mulated an amount equal to some integral multiple of hv. 

 A form of the theory which does not contain this improb- 

 able contradiction of the firmly established facts of 

 electrodynamics introduces the quantum into the specifi- 

 cation of the energy of vibration which is permitted to 

 each oscillator. Here both emission and absorption fol- 

 low the classical theory, but the motion of an emitting 

 and absorbing linear oscillator of frequency v is supposed 

 to be stable only for those amplitudes for which the energy 

 of its oscillations is an integral multiple of hv. In order 

 to maintain the energy at these particular values, the 

 oscillator may draw energy from, or deposit surplus 

 energy with, other degrees of freedom which partake 

 neither in emission nor absorption, but act merely as 

 storehouses. . 



Photo-electric Effect. — "When investigating the produc- 

 tion of electromagnetic waves, Hertz had noticed that a 

 spark passed more readily between the terminals of his 

 oscillator when the negative electrode was illuminated by 

 light from another spark. Further investigation by 

 Hallwachs, Elster and Geitel, and others showed that this 

 effect was due to the emission of electrons by a metal 

 exposed to the influence of ultra-violet light. Lenard 



