An example of a historical scientific idea that would now be considered an alternative conception.
Considerable debate went into understanding the nature and properties of the ether that pervades all space (and is no longer considered to exist):
"the apparent laws of reflection and of refraction are not altered by the Earth's motion. This phenomenon admits of two explanations.
We may suppose that the ether is not in repose, but that it is displaced by bodies in motion. It would not then be astonishing that the phenomenon of refraction should not be altered by the Earth's motion, since everything-lenses, telescopes, and ether-would be carried along together by the same motion. As for aberration itself, it would be explained by a kind of refraction produced at the surface of separation of the ether in repose in the interstellar spaces and the ether carried along by the Earth's movement. It is upon this hypothesis (the total translation of the ether) that Hertz's theory of the Electro-dynamics of bodies in motion is founded.
Fresnel, on the contrary, supposes that the ether is in absolute repose in space, and almost in absolute repose in the air, whatever be the velocity of that air, and that it is partially displaced by refringent mediums. Lorentz has given this theory a more satisfactory form. In his view the ether is in repose and the electrons alone are in motion. In space, where the ether alone comes into play, and in the air, where it comes almost alone into play, the displacement is nil or almost nil. In refringent mediums, where the perturbation is produced both by the vibrations of the ether and by those of the electrons set in motion by the agitation of the ether, the undulations are partially carried along….
But if the ether is not displaced by the Earth's motion, is it possible by means of optical phenomena to demonstrate the absolute velocity of the Earth, or rather its velocity in relation to the motionless ether? …
Is the action of the Maxwell-Bartholi pressure upon the matter of the transparent medium equal to its reaction upon the source, and that, whatever that matter may be? Or rather, is the action less in proportion as the medium is less refringent and more rarefied, becoming nil in a vacuum? If we admit Hertz's theory, which regards the ether as mechanically attached to matter, so that the ether is completely ·carried along by matter, we must answer the first and not the second question in the affirmative."
Henri Poincaré (1914) Science and Method (trans. Francis Maitland) Dover Publications, 1952.
