194 SCIENCE PROGRESS 



alike in geometrical pattern, so that L/C is the same for all 

 vibrators. Now it is a fact which is worthy of notice that the 

 quantity 



eVETc 



is one whose dimensions are the same in all possible systems of 

 electromagnetic units ; its dimensions are in fact those of 

 Action. Since the assumption of similar geometric pattern 

 leads to identical values for L/C for all atoms, one is encouraged 

 to assume that e'\/L/C is a universal constant, and it appears 

 on investigation that the energy first absorbed and now radiated 

 by the discharging oscillator is connected with the frequency 

 V above by the equation 



U = Tre VL/C . V 



which is just Planck's equation if we identify h with 7re*-\/L/C. 



It appears that Whittaker's rather cavalier manner of 

 treating the model at the end raised some criticism in the 

 discussion which followed his paper ; he, nevertheless, defends 

 his attitude in some characteristic remarks which deserve 

 quotation, so important are they to a philosophical view of the 

 development of Physical Science : "At the same time," he 

 says, " we must bear in mind that the function of models is 

 merely to suggest the correct differential equations of the 

 phenomenon : when the differential equations have been 

 obtained the model may be discarded. Instances of this in the 

 history of Physics are abundant : to name only the most 

 famous of them, it was a model of rolling particles, idle wheels, 

 and cellular vortices that suggested to Maxwell the correct 

 differential equations of the ether. The model, having served 

 its purpose, soon dropped out of sight : and the ether itself 

 appears to be following it into oblivion. The differential 

 equations alone remain, and in the hands of the relativists have 

 provided the foundation for a complete reconstruction of our 

 ideas of the universe." 



However, it is not impossible to retain the model. Whit- 

 taker, himself, in a note added to his paper before publication, 

 points out that if we add a circuit of wire (supposed perfectly 

 conducting) linked once with the rotating magnetic structure 

 we obtain the necessary oscillations in the magnet's motion 

 and the current induced in the wire to set up radiation. But, 

 probably, those interested in models will prefer Ewing's sugges- 

 tion, in which he pointed out that Whittaker had made use only 

 of the interior, movable part of the new ferromagnetic model. 

 If one does not forget the outer stationary part, it appears that 

 when the critical energy is communicated to the rotating part 



