—which is to be conveniently supposed at rest relatively to the corresponding co-ordinate system at the moment of departure of the ray—in the direction of the
-axis for a distance
to a point
, then an observer at
will, owing to his own acceleration,
—which is to be conveniently supposed at rest relatively to the corresponding co-ordinate system at the moment of departure of the ray—in the direction of the
-axis for a distance
to a point
, then an observer at
will, owing to his own acceleration,