MOTION THROUGH THE ETHER.

The Michelson-Morley Experiment.

When we have spoken of a system at rest we have so far meant, for all practical purposes, a system at rest in our laboratories. But if we have been right in conjecturing that all electromagnetic phenomena have their seat in the ether, then a system at rest would most naturally be taken to mean a system at rest in the ether. We have so far made no clear distinction between the conceptions of rest in the ether and rest relative to the walls of a laboratory.

The view was at one time held that a moving body drags the ether along with it. If this were a true view the distinction just referred to would not arise; a body at rest relative to the walls of a laboratory would also be at rest in the ether. But in time it was found that this was not a true view; it could not be reconciled simultaneously with results of laboratory experiments such as Fizeau's water-tube experiment (cf. § 687 below), and with the astronomical theory of the aberration of light (cf. § 689 below). Finally it became established that the ether, if one existed at all, could not share in the motion of moving bodies ; it must be stagnant, and moving bodies must simply move through it without setting up mass-motions in it.