ESSAYS 477 



by an arrangement of additional mirrors reflecting the rays backwards and forwards. 

 The apparatus would have indicated a displacement of about a twentieth of the 

 amount expected, but the result was entirely negative. The expected retardation 

 in time was rather less than the thousand-million-millionth of a second. 



If, however, there were no drift through the ether, it would follow that the 

 earth carried a layer of ether along with it, and if this were the case, so also would 

 the other planets. But this would necessarily give rise to eddies in the ether, and 

 other optical results showed that its motion must be entirely free from eddies, or, 

 in other words, free from any trace of spin. 



It was suggested, almost simultaneously, by Fitzgerald and by Lorentz, that 

 the null result would be accounted for if the whole apparatus, in common with all 

 solid bodies moving through the ether, underwent an extremely minute contraction 

 in the direction of motion, of an amount which, for speeds comparable to that of 

 the earth's orbital motion, would be proportional to the square of the speed, and 

 for this speed would amount to about one part in two hundred millions, or about 

 65 centimetres in the earth's diameter. This seemed a startling proposition, but 

 reason was shown for anticipating some contraction, and a little later, some new 

 observations led Lorentz and Larmor by two very different paths, neither of which 

 had any apparent relation to the Michelson-Morley experiment, to the conclusion 

 that such contraction must occur, and that its amount would be exactly that 

 required to explain the null effect. But other attempts, made in very diverse 

 ways, to obtain evidence of the motion of material bodies through the ether were 

 all defeated by some corresponding compensation, just as though the forces of 

 nature were in a conspiracy to prevent our attainment of the evidence sought for. 



It was therefore suggested by some physicists that, under the title of the 

 principle of relativity, it should be assumed as an axiom that it is inherently 

 impossible to obtain experimental evidence of the motion of material bodies 

 relatively to the ether. In order to form a starting-point for physical argument, 

 this hypothesis was embodied in a statement to the effect that to any observer, 

 whatever his own motion might be, light will appear to travel through space, that 

 is to say, through ether free from matter, always in straight lines and always at 

 one and the same speed. Some out-and-out relativists even proposed to discard 

 the ether, as a mere framework existing only in our own minds, mainly on the 

 ground that it would simplify the fundamental differential equations of electro- 

 dynamics. But this simplification would be obtained at the expense of reducing 

 their content, in that it would leave out of account the existence of so firmly 

 established a phenomenon as the inertia of free-travelling radiation, exhibited in 

 the pressure exerted by a ray of light upon any material body upon which it 

 impinges. And the existence of such a pressure is a direct consequence of the 

 universally accepted Faraday- Maxwell theory, which led to its being sought for 

 and experimentally verified by Lebedeff. Moreover, the discarding of the ether, 

 by making anything in the shape of a mechanical interpretation of electrical action 

 impossible, would render the interpretation of these simplified differential 

 equations in terms of definite physical concepts almost, if not altogether, impossible, 

 and differential equations, although most valuable guides in the interpretation of 

 nature, can hardly be regarded as in themselves constituting such interpretation. 

 The elimination of the ether from our representations of physical phenomena 

 would have the further effect of reopening the age-long controversy between 

 Newton's relativist critics and the supporters of his view that absolute motion in 

 space, and hence also absolute direction in space, are legitimate physical concepts. 



According to the older relativity theory, the Copernican system of astronomy 



