If A and B are two points at rest relative to the coordinate
system, which are equipped with clocks and are separated by a distance r, while t A is the
reading of the clock at A at the moment when a ray of light propagating through a vacuum
in the direction AB reaches point A, and t B is the reading of the clock at Bat the moment
the ray reaches B, then we should always have r/(tB–tA)¼c, whatever the motion of the
light source emitting the light ray or the motion of the other bodies may be.’’