, the postulate of the invariant velocity of light demands that the distance
measured by us in the frame, divided by the duration required (according to the time of our frame) for a light wave to cover this distance, be always equal to
, the velocity of light. If, now, we had viewed the same phenomenon from some other Galilean system, the points
and
of the first system at the instants when the ray of light passed them would have corresponded to some other points