c2 - v2
Now the ray of light moves relative to the origin of k with a velocity c - v, measured in the stationary system; therefore we have
x′
-------- = t
c - v
Substituting these values of t in the equation for ξ, we obtain
c2
ξ = a ------------ x′
c2 - v2
In an analogous manner, we obtain by considering the ray of light which moves along the y-axis,