As constructed by Fresnel, the hypothesis of the luminous ether, which had so great a struggle at the outset to overcome the stubborn resistance of the partisans of the then classic theory of emission, seemed, on the contrary, to possess in the sequel an unshakable strength.  Lame, though a prudent mathematician, wrote:  “The existence of the ethereal fluid is incontestably demonstrated by the propagation of light through the planetary spaces, and by the explanation, so simple and so complete, of the phenomena of diffraction in the wave theory of light”; and he adds:  “The laws of double refraction prove with no less certainty that the ether exists in all diaphanous media.”  Thus the ether was no longer an hypothesis, but in some sort a tangible reality.  But the ethereal fluid of which the existence was thus proclaimed has some singular properties.

Were it only a question of explaining rectilinear propagation, reflexion, refraction, diffraction, and interferences notwithstanding grave difficulties at the outset and the objections formulated by Laplace and Poisson (some of which, though treated somewhat lightly at the present day, have not lost all value), we should be under no obligation to make any hypothesis other than that of the undulations of an elastic medium, without deciding in advance anything as to the nature and direction of the vibrations.

This medium would, naturally—­since it exists in what we call the void—­be considered as imponderable.  It may be compared to a fluid of negligible mass—­since it offers no appreciable resistance to the motion of the planets—­but is endowed with an enormous elasticity, because the velocity of the propagation of light is considerable.  It must be capable of penetrating into all transparent bodies, and of retaining there, so to speak, a constant elasticity, but must there become condensed, since the speed of propagation in these bodies is less than in a vacuum.  Such properties belong to no material gas, even the most rarefied, but they admit of no essential contradiction, and that is the important point.[20]

[Footnote 20:  Since this was written, however, men of science have become less unanimous than they formerly were on this point.  The veteran chemist Professor Mendeleeff has given reasons for thinking that the ether is an inert gas with an atomic weight a million times less than that of hydrogen, and a velocity of 2250 kilometres per second (Principles of Chemistry, Eng. ed., 1905, vol. ii. p. 526).  On the other hand, the well-known physicist Dr A.H.  Bucherer, speaking at the Naturforscherversammlung, held at Stuttgart in 1906, declared his disbelief in the existence of the ether, which he thought could not be reconciled at once with the Maxwellian theory and the known facts.—­ED.]