An example of a historical scientific idea that would now be considered an alternative conception:
"All these electrons are immersed in ether. The ether is everywhere identical with itself, and perturbations are produced in it, following the same laws as light or the Hertzian oscillations in empty space. Beyond the electrons and the ether there is nothing. When a luminous wave penetrates a part of the ether where the electrons are numerous, these electrons are set in motion under the influence of the perturbation of the ether, and then react upon the ether. This accounts for refraction, dispersion, double refraction, and absorption. In the same way, if an electron was set in motion for any reason, it would disturb the ether about it and give birth to luminous waves, and this explains the emission of light by incandescent bodies."
Henri Poincaré (1914) Science and Method (trans. Francis Maitland) Dover Publications, 1952.
