152. AMBIGUITY OF THE WORD "LIGHT": Natural Agency vs. Human Impression; The Luminiferous Ether.
The word "light" mayĀ be used in two different senses; it may meanĀ the impression made upon consciousness, or itĀ may mean the physical agent which makes theĀ impression. . . . That agent is a substanceĀ which fills all space, and surrounds the atomsĀ and molecules of bodies. To this interstellarĀ and interatomic medium definite mechanicalĀ properties are ascribed, and we deal with itĀ in our reasonings and calculations as a bodyĀ possessed of these properties. In mechanics weĀ have the composition and resolution of forcesĀ and of motions, extending to the compositionĀ and resolution of vibrations. We treat theĀ luminiferous ether on mechanical principles,Ā and, from the composition, resolution, andĀ interference of its vibrations we deduce allĀ the phenomena displayed by crystals inĀ polarized light.
ā TYNDALL Lectures on Light,Ā 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.