_c²_
ξ = _a_ ------------- _x′_
_c²_ - _v²_
In an analogous manner, we obtain by considering the ray of light which
moves along the _y_-axis,
_vx′_
η = _c_τ = _a c_(_t_ - ------------- )
_c²_ - _v²_
where
_y_
------------------ = _t_, _x′_ = 0,
√ (_c²_ - _v²_)