- A SKEPTICs GUIDE - A SKEPTICs GUIDE 
This sort of nonsense convinced most people that MAXWELL'S EQUATIONS were wrong -- or, more charitably, incomplete. The obvious way out of this dilemma was to assume that what we perceive (in our ignorance) as vacuum is actually an extremely peculiar substance called the ``luminiferous æther'' through which ordinary ``solid'' matter passes more or less freely but in which the ``field lines'' of electromagnetism are actual ``ripples.'' (Sort of.) This recovers the rationalizing influence of a medium through which light propagates, at the expense of some pretty unfamiliar properties of the medium. [You can see the severity of the dilemma in the lengths to which people were willing to go to find a way out of it.] All that remained was to find a way of measuring the observer's velocity relative to the æther.
Since ``solid'' objects slip more or less effortlessly through the æther, this presented some problems. What was eventually settled for was to measure the apparent speed of light propagation in different directions; since we are moving through the æther, the light should appear to propagate more slowly in the direction we are moving, since we are then catching up with it a little.23.2