The speed of light, c, is the same for all inertial observers but no one has ever actually measured it in one direction. So taking an normal emitter, reflector, detector setup, can we do the following? Make the first measurement, and log the time. Put a medium that allows light through but "slows" it down slightly, put in front of the emitter side, take another measurement. Then take that same medium and put it in front of the detector side, take the last measurement, and compare. Test a should give us the same we always get. If light moves the same velocity in every direction, test b and c should give the same result, if not then they will be different, meaning light changes velocity depending on the direction it is going.
If I am wrong, please tell me what is wrong and why. What are your thoughts on trying to measure the one-way speed of light, and do you think it is possible at all?