152. AMBIGUITY OF THE WORD "LIGHT": Natural Agency vs. Human Impression; The Luminiferous Ether.
The word "light" may be used in two different senses; it may mean the impression made upon consciousness, or it may mean the physical agent which makes the impression. . . . That agent is a substance which fills all space, and surrounds the atoms and molecules of bodies. To this interstellar and interatomic medium definite mechanical properties are ascribed, and we deal with it in our reasonings and calculations as a body possessed of these properties. In mechanics we have the composition and resolution of forces and of motions, extending to the composition and resolution of vibrations. We treat the luminiferous ether on mechanical principles, and, from the composition, resolution, and interference of its vibrations we deduce all the phenomena displayed by crystals in polarized light.
— TYNDALL Lectures on Light, lect. 4, p. 128. (A., 1898.)
Blogger's note: So, light is sort of a wave and sort of a particle (or maybe sometimes one and sometimes the other). When people thought it was only a wave, they figured that there had to be some medium for the light-wave to propagate through (since a wave is a disturbance that moves through a medium). Scientists then named that hypothetical medium the luminiferous ether. It isn't thought to actually exist anymore -- instead, we have relativity and quantum theory to help explain it away.