, taking
seconds to reach M. On the way back, O will be advancing to meet it, and the return trip will occupy
seconds. The elapsed time for the round trip comes out
seconds, which is longer than when the system was at rest—the loss of time in the “stern chase” exceeding the saving on the return.
The light which is reflected from N has a different history. When it starts, O and N have certain positions in the ether,
