THE RELATIVITY OF SPACE. 97 



but we can also measure a distance by the time that 

 hght takes to traverse it, on condition that we admit 

 that the velocity of light is constant, and independent 

 of its direction. Lorentz could have accounted for the 

 facts by supposing that the velocity of light is greater 

 in the direction of the earth's motion than in the 

 perpendicular direction. He preferred to admit that 

 the velocity is the same in the two directions, but that 

 bodies are smaller in the former than in the latter. If 

 the surfaces of the waves of light had undergone the 

 same deformations as material bodies, we should never 

 have perceived the Lorentz-Fitzgerald deformation. 



In the one case as in the other, there can be no 

 question of absolute magnitude, but of the meas- 

 urement of that magnitude by means of some instru- 

 ment. This instrument may be a yard-measure or 

 the path traversed by light. It is only the relation 

 of the magnitude to the instrument that we measure, 

 and if this relation is altered, we have no means of 

 knowing whether it is the magnitude or the instrument 

 that has changed. 



But what I wish to make clear is, that in this 

 deformation the world has not remained similar to 

 itself Squares have become rectangles or parallel- 

 ograms, circles ellipses, and spheres ellipsoids. And 

 yet we have no means of knowing whether this de- 

 formation is real. 



It is clear that we might go much further. Instead 

 of the Lorentz-Fitzgerald deformation, with its ex- 

 tremely simple laws, we might imagine a deformation 

 of any kind whatever ; bodies might be deformed in 

 accordance with any laws, as complicated as we liked, 

 and we should not perceive it, provided all bodies 



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