Well, then, perhaps it is stagnant. The experiments I have quoted do not prove that it is so. They are equally consistent with its perfect freedom and with its absolute stagnation; though they are not consistent with any intermediate position. Certainly, if the ether were stagnant nothing could be simpler than their explanation.
The only phenomena then difficult to explain would be those depending on light coming from distant regions through all the layers of more or less dragged ether. The theory of astronomical aberration would be seriously complicated; in its present form it would be upset (p. [45]). But it is never wise to control facts by a theory; it is better to invent some experiment that will give a different result in stagnant and in free ether. None of those experiments so far described are really discriminative. They are, as I say, consistent with either hypothesis, though not very obviously so.
Fig. 10. The course of the light and of the two half-beams in Michelson's most famous experiment.
The light is split at A, one half sent towards B and back, the other half to C and back. Compare with Fig. [7].
Michelson Experiment.
Mr. Michelson, however, of the United States, invented a plan that looked as if it really would discriminate; and, after overcoming many difficulties, he carried it out. It is described in the Philosophical Magazine for 1887.
Michelson's famous experiment consists in looking for interference between two half-beams of light, of which one has been sent to and fro across the line of ether drift, and the other has been sent to and fro along the line of ether drift.
A semi-transparent mirror set at 45° is employed to split the beam, and a pair of normal and ordinary mirrors, set perpendicular to the two half-beams, are employed to return them back whence they came, so that they can enter the eye through an observing telescope.