EM AN CI PA TION FROM SCIENTIFIC MA TERIALISM. 425 



exception in some unexplainable contradiction between 

 what really happens, and what we should expect from our 

 mechanical model. This contradiction may long remain 

 hidden ; but the history of science teaches us that it sooner 

 or later makes its inevitable appearance, and that all we can 

 say with complete, certainty regarding such mechanical 

 similes or analogues — usually termed mechanical theories 

 of the phenomena in question — is that they will doubtless 

 on some occasion fail. 



The history of optical theories offers a striking example 

 of these facts. So long as the whole of optics embraced 

 only reflection and refraction, its phenomena could be 

 represented by the mechanical system proposed by Newton, 

 according to which light consisted of small particles which 

 were shot out in straight lines from the radiating object, 

 and obeyed the laws of motion for perfectly elastic bodies. 

 The fact that another mechanical view, the vibration theory 

 of Huygens and Euler, yielded just as much in these 

 respects might well have awakened doubts as to the 

 exclusive ability of the earlier hypothesis to meet the 

 requirements of the case, but it was not able to usurp the 

 latter's scientific position. When, however, the phenomena 

 of interference and polarisation were discovered, Newton's 

 mechanical analogue showed itself to be quite unsuitable, 

 and the other, namely, the vibration theory, was considered 

 established, since from its assumptions the chief points at 

 least of the new phenomena could be deduced. 



The life of the vibration theory as a mechanical theory 

 has also had its bounds, for in our own time it too has been 

 carried to its grave without drum or fife, and its place 

 taken by the electro-magnetic theory of light. A post- 

 mortem examination reveals clearly the cause of death ; 

 it has resulted from the failure of the mechanical parts. 

 The hypothetical ether whose task it was to " beat" had to 

 fulfil its duty under particularly difficult conditions. For 

 the phenomena of polarisation demanded unconditionally 

 that the vibrations should be transversal. Now this pre- 

 supposes a solid body, and the calculations of Lord Kelvin 

 have finally shown that a medium with such properties as 



