The relations discussed so far can easily be extended from two to several observers. Doing this, first the addition
of velocities must be derived, because the relativistic case shows not the simple summation which could be expected
according to the laws of the Galilei-Transformation. Further special relations exist in connection with velocities
lower than the speed of light, which are observed e.g. concerning light in transparent media or connected with the
transport of sound in solid bodies. These relations are also valid during acceleration of observers, because material
objects cannot be considered as absolute rigid.
In addition the case is discussed, when the transport of a signal inside a moving body is not only taking place in
the direction of the movement but also transverse to it.