We know that ether was, at first, only one of the numerous fluids of physics, but with the new experiments having proved to Fresnel that light waves are transversal, we had to create a body analogous to elastic solids. But then, how can the other bodies move through it without experiencing any appreciable resistance? The question was all the more difficult with the problem of aberration obliging us to

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admit that the ether does not participate in the earth's translational motion, so that all bodies are constantly traversed by a current of ether of 30 km per second, with zero effects, despite the rigidity of ether. We must add that the elasticity of this body is quite singular, seeing that its resistance to compression would be zero, which wouldn't happen for a finite solid. We can, it is true, appeal to Lord Kelvin's rotational elasticity, by being (Oeuvres 368) careful not to take into account the perturbation that would be brought to this ingenious mechanism by the brutal passage of an animated body with a speed of 30 km per second.
      The difficulties increased when, the identity of light and electrical oscillations having been shown, we had to extend this system of explanations to electromagnetism as a whole. Poincaré has exposed some of the strangeness to which we are led. Besides, experiments have refused to accord to ether that primordial property of bodies, movement, Fizeau's experiments (interpreted by Lorentz), and those of Lodge, and others, have concurred in their negative results: ether is not entrained by the movement of matter, nor by that of charged or magnetized bodies, nor by currents, etc. The hypothesis itself, of such motions, does not permit obtaining a mechanical explanation of electrodynamics. We resigned ourselves to accept the absolute rest of ether; the hypothesis of a complete compenetration permitted the avoidance of the difficulty relative to the movement of bodies through ether. This latter has become what Drude calls a "Physical Space", the seat of electric and magnetic energy, and polarizations. It furnishes a system of coordinates independent of all

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matter, and to which Maxwell's and Lorentz's equations must be referred.
      This is already too much abstraction. That still is not enough. Indeed, according to these views, ether could also be the seat of phenomena independent of matter, and thus manifest its existence. Nothing of the kind; and, in order to explain it, we needed a new hypothesis, discarding all waves that don't diverge from a material element of volume. The role of ether is again reduced. We have seen that, from now on, we can completely set aside the notion of field and the consideration of what is passing in the ether, and to be content with elementary actions of charges on each other (exactly as In the older theories of Gauss, Weber, Riemann and Clausius, but with a finite time of transmission). We thus express the same facts, but by including the hypothesis on the divergence of waves and the consequential (Oeuvres 369) irreversibility, and which the equations of partial derivatives are powerless to express. The ether has become a system of absolute coordinates, a mathematical abstraction; the equations of partial derivatives, an intermediate mental construct which, however, isn't sufficient in itself.
      Finally, this phantom of ether itself did not stand up under the scrutiny of experiment. It seems well accepted that absolute motion cannot be put into evidence. We have seen to which hypotheses, disrupting all the principles of physics, we must have recourse to, to render account of this result. The only conclusion which, from then on, seems possible to me, is that ether doesn't exist, or more exactly, that we should renounce use of this representation that the motion of light is a relative motion like all the others, that only relative velocities play a role in the laws of nature; and finally that we should renounce use of partial differential equations

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and the notion of field, in the measure that this notion introduces absolute motion.
      As I already stated in the Introduction, this overly negative conclusion needs two complimentary remarks; a simple representation for the new mode of light motion; and the demonstration that a theory satisfying these principles is possible.
      The habit that we have to "substantiate", if I dare to express myself so, a habit which we owe to the old caloric, magnetic, etc., fluids and the new energy fluid, makes indispensable, indeed, the introduction of a representation which makes us realize what happens to light and electrical forces when, having left a body, they don't act on still another. A theory which wouldn't admit such a representation would be considered by many as introducing actions at a distance, simply retarded. Moreover, as Poincaré noted ( Science et Hypothesis, p. 199) [1], and this is one of the reasons that we can judge in favor about the existence of ether. Mechanics would have it that the state of a system depends only on the immediately preceding states. It wouldn't be so anymore if we canceled all intermediates. Actually, we thus save only a convention, which perhaps doesn't have any extreme usefulness. We have seen that we can't arbitrarily give the initial state of ether, which must (Oeuvres 370) satisfy the formulae of retarded potentials. It is to say that the consideration of the system during a finite period is not avoided effectively. On the other hand, the pressure exerted by light on a mirror, even in a vacuum, for example, is contrary to the equality of action and reaction when applied to the material alone. We will therefore have to "substantialize" the radiant energy to save the principle and that of the conservation of energy whenever there is a body in which the radiation doesn't meet any material obstacle in certain

[1] This is probably page 119. Ed.

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directions, and for which the energy cannot, consequently, ever be fully recovered. These principles will then become conventions, in part at least, but for a greater advantage to the economy of our thought.