Aided by the following illustration, we can readily see that, in
accordance with the Lorentz transformation, the law of the
transmission of light in vacuo is satisfied both for the
reference-body K and for the reference-body K1. A light-signal is sent
along the positive x-axis, and this light-stimulus advances in
accordance with the equation

x = ct,

i.e. with the velocity c. According to the equations of the Lorentz
transformation, this simple relation between x and t involves a
relation between x1 and t1. In point of fact, if we substitute for x
the value ct in the first and fourth equations of the Lorentz
transformation, we obtain:

eq. 3: file eq03.gif


eq. 4: file eq04.gif

from which, by division, the expression

x1 = ct1

immediately follows. If referred to the system K1, the propagation of
light takes place according to this equation. We thus see that the
velocity of transmission relative to the reference-body K1 is also
equal to c. The same result is obtained for rays of light advancing in
any other direction whatsoever. Of cause this is not surprising, since