shall not enter explicitly into the formulas to be developed later. Then the Lorentz transformation is defined in such a way that, first, it makes the equation
a co-variant equation, that is, an equation which is satisfied with respect to every inertial system if it is satisfied in the inertial system to which we refer the two given events (emission and reception of the ray of light). Finally, with Minkowski, we introduce in place of the real time co-ordinate
, the imaginary time co-ordinate
Then the equation defining the propagation of light, which must be co-variant with respect to the Lorentz transformation, becomes
This condition is always satisfied[8] if we satisfy the more general condition that
