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In one of the most notable of these attempts Michelson devised a

method which appears as though it must be decisive. Imagine two

mirrors so arranged on a rigid body that the reflecting surfaces

face each other. A ray of light requires a perfectly definite

time T to pass from one mirror to the other and back again, if

the whole system be at rest with respect to the aether. It is

found by calculation, however, that a slightly different time T′

is required for this process, if the body, together with the

mirrors, be moving relatively to the aether. And yet another

point: it is shown by calculation that for a given velocity v

with reference to the aether, this time T′ is different when the

body is moving perpendicularly to the planes of the mirrors from

that resulting when the motion is parallel to these planes.