Explanatory Notes 121 



theory would have to be overhauled. Moreover, my 

 experiment with steel disks, designed to detect a trace of 

 viscosity, failed to show any: in fact negatived the idea. 

 So the FitzGerald-Lorentz hypothesis was devised in 

 order to explain the negative result of Professor Michelson 

 in another way, by postulating a minute change of shape 

 or distortion of all bodies as they move through the Ether 

 of space at any excessive speed ; and there is a great deal 

 to be said in favour of such a hypothesis. Some physicists, 

 however, consider it only a hyper-ingenious and imagin- 

 ative device of evading awkward and irreconcilable facts. 

 Many other experiments have been made to detect the 

 effect of an etherial movement relatively to the earth; 

 and they have all uniformly failed. So at length Professor 

 Einstein first, and now many others, have supposed that 

 the Universe is so adjusted that an observation of this 

 kind is for ever impossible. They boldly make the postu- 

 late or axiom that although motion of matter with respect 

 to matter is readily perceived, motion of matter with re- 

 spect to ether is impossible to observe and is in fact mean- 

 ingless. They formulate the principle that nothing 

 but relative motion of pieces of matter with respect-to 

 each other can ever be detected, and that no change 

 in the velocity of light can ever be observed except when 

 there is relative motion of matter. This principle, en- 

 throned as the Principle of Relativity, has become the 

 foundation of a mathematical erection, with far-reaching 

 and in some cases surprising and almost paradoxical con- 

 sequences, affecting the going of clocks and even the nature 

 of Time. 



The difficult part of the theory of Relativity is connected 

 with the deduction of all these remarkable consequences: 

 the fact that motion with respect to the ether is difficult 

 to observe, together with the familiar fact of the extreme 

 ease of observing the relative motion of pieces of matter 

 with respect to each other, constitutes the superficial and 



